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Cambridge IGCSE · 0620

Chemistry

Stoichiometry — Topics 1 and 4

Name: ____________________Date: October 10, 2026
  1. 1.

    Calculate the number of moles in 5.85 g of sodium chloride, NaCl. Use Ar: Na = 23.0, Cl = 35.5.

    [3 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. One NaCl formula unit contains one sodium atom and one chlorine atom. Add the given relative atomic masses: Mr = 23.0 + 35.5 = 58.5.
    2. The molar mass is therefore 58.5 g/mol. Divide mass by molar mass: n = 5.85 g / (58.5 g/mol) = 0.100 mol.
    3. Relative formula mass has no unit; molar mass has g/mol. Multiplying 0.100 mol by 58.5 g/mol returns the given 5.85 g, checking the result.

    Marking points

    • Calculates Mr(NaCl) = 23.0 + 35.5 = 58.5.
    • Uses amount = mass ÷ relative formula mass.
    • Obtains 0.100 mol.

    Examiner tip: Add the relative atomic masses before dividing the sample mass.

  2. 2.

    Magnesium reacts with hydrochloric acid: Mg + 2HCl → MgCl₂ + H₂. Identify the limiting reactant when 0.10 mol Mg reacts with 0.15 mol HCl, and calculate the amount of hydrogen formed.

    [4 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Work through this mathematical step: Uses the equation ratio 1 mol Mg : 2 mol HCl. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Develop this part of the answer: Recognises that 0.10 mol Mg would require 0.20 mol HCl. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: Identifies HCl as the limiting reactant. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Work through this mathematical step: Obtains n(H₂) = 0.15 ÷ 2 = 0.075 mol. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Compare available amounts only after applying the equation coefficients.

    Marking points

    • Uses the equation ratio 1 mol Mg : 2 mol HCl.
    • Recognises that 0.10 mol Mg would require 0.20 mol HCl.
    • Identifies HCl as the limiting reactant.
    • Obtains n(H₂) = 0.15 ÷ 2 = 0.075 mol.

    Examiner tip: Compare available amounts only after applying the equation coefficients.

  3. 3.

    Calculate the relative formula mass (Mr) of calcium carbonate, CaCO₃. Use Ar: Ca = 40, C = 12, O = 16.

    [2 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Develop this part of the answer: Adds the relative atomic masses correctly, including three oxygens: 40 + 12 + (3×16). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Work through this mathematical step: Obtains Mr = 100. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Multiply the atomic mass of an element by its subscript in the formula before adding it to the total.

    Marking points

    • Adds the relative atomic masses correctly, including three oxygens: 40 + 12 + (3×16).
    • Obtains Mr = 100.

    Examiner tip: Multiply the atomic mass of an element by its subscript in the formula before adding it to the total.

  4. 4.

    Calculate the percentage by mass of oxygen in magnesium oxide, MgO. Use Ar: Mg = 24, O = 16.

    [3 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Work through this mathematical step: Calculates Mr(MgO) = 24 + 16 = 40. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Work through this mathematical step: Uses percentage = (mass of oxygen ÷ Mr) × 100 = (16/40) × 100. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Develop this part of the answer: Obtains 40%. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Percentage composition always divides the mass of the element in question by the total formula mass, not by the mass of any other element.

    Marking points

    • Calculates Mr(MgO) = 24 + 16 = 40.
    • Uses percentage = (mass of oxygen ÷ Mr) × 100 = (16/40) × 100.
    • Obtains 40%.

    Examiner tip: Percentage composition always divides the mass of the element in question by the total formula mass, not by the mass of any other element.

  5. 5.

    Calculate the volume, in dm³, occupied by 0.50 mol of carbon dioxide gas at room temperature and pressure (rtp), given that 1 mole of gas occupies 24 dm³ at rtp.

    [2 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Work through this mathematical step: Uses volume = number of moles × 24 dm³. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Work through this mathematical step: Obtains volume = 12 dm³. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: The molar gas volume (24 dm³/mol at rtp) applies to any gas, regardless of its identity, since it depends only on the number of particles.

    Marking points

    • Uses volume = number of moles × 24 dm³.
    • Obtains volume = 12 dm³.

    Examiner tip: The molar gas volume (24 dm³/mol at rtp) applies to any gas, regardless of its identity, since it depends only on the number of particles.

  6. 6.

    Calculate the empirical formula of a compound containing 40% carbon, 6.7% hydrogen and 53.3% oxygen by mass. Use Ar: C = 12, H = 1, O = 16.

    [4 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Work through this mathematical step: Divides each percentage by its relative atomic mass: C = 40/12 = 3.33, H = 6.7/1 = 6.7, O = 53.3/16 = 3.33. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Work through this mathematical step: Divides each result by the smallest value (3.33): C = 1, H = 2.01, O = 1. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Work through this mathematical step: Rounds to whole-number ratios: C:H:O = 1:2:1. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    5. Work through this mathematical step: States the empirical formula as CH₂O. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Always divide by the atomic mass first (to get moles of each element), then divide by the smallest mole value to get the simplest whole-number ratio.

    Marking points

    • Divides each percentage by its relative atomic mass: C = 40/12 = 3.33, H = 6.7/1 = 6.7, O = 53.3/16 = 3.33.
    • Divides each result by the smallest value (3.33): C = 1, H = 2.01, O = 1.
    • Rounds to whole-number ratios: C:H:O = 1:2:1.
    • States the empirical formula as CH₂O.

    Examiner tip: Always divide by the atomic mass first (to get moles of each element), then divide by the smallest mole value to get the simplest whole-number ratio.

  7. 7.

    A student dilutes 20.0 cm³ of 2.0 mol/dm³ hydrochloric acid to a total volume of 100 cm³. Calculate the new concentration of the diluted acid.

    [3 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Work through this mathematical step: Calculates the original moles of acid: n = 2.0 × 0.0200 = 0.0400 mol. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Develop this part of the answer: States that the number of moles of acid is unchanged by dilution. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Work through this mathematical step: Uses new concentration = moles ÷ new volume (dm³) = 0.0400/0.100 to obtain 0.40 mol/dm³. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Dilution never changes the number of moles of solute — only the volume (and hence concentration) changes.

    Marking points

    • Calculates the original moles of acid: n = 2.0 × 0.0200 = 0.0400 mol.
    • States that the number of moles of acid is unchanged by dilution.
    • Uses new concentration = moles ÷ new volume (dm³) = 0.0400/0.100 to obtain 0.40 mol/dm³.

    Examiner tip: Dilution never changes the number of moles of solute — only the volume (and hence concentration) changes.

  8. 8.

    State Avogadro's constant and use it to calculate the number of molecules in 0.25 mol of water.

    [3 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Develop this part of the answer: States Avogadro's constant as 6.02 × 10²³ per mole. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Work through this mathematical step: Uses number of molecules = moles × Avogadro's constant. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Develop this part of the answer: Obtains 1.51 × 10²³ molecules. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Avogadro's constant links the mole to a count of actual particles — always multiply (not divide) moles by it to get the number of particles.

    Marking points

    • States Avogadro's constant as 6.02 × 10²³ per mole.
    • Uses number of molecules = moles × Avogadro's constant.
    • Obtains 1.51 × 10²³ molecules.

    Examiner tip: Avogadro's constant links the mole to a count of actual particles — always multiply (not divide) moles by it to get the number of particles.

  9. 9.

    Sodium carbonate reacts with hydrochloric acid: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. Calculate the mass of sodium carbonate needed to react exactly with 0.20 mol of hydrochloric acid. Use Ar: Na = 23, C = 12, O = 16.

    [4 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Work through this mathematical step: Uses the 1:2 equation ratio to find n(Na₂CO₃) = 0.20 ÷ 2 = 0.10 mol. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Work through this mathematical step: Calculates Mr(Na₂CO₃) = (2×23) + 12 + (3×16) = 106. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Work through this mathematical step: Uses mass = moles × Mr. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    5. Work through this mathematical step: Obtains mass = 0.10 × 106 = 10.6 g. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Always convert to moles first using the balanced equation ratio before converting back to mass with the relative formula mass.

    Marking points

    • Uses the 1:2 equation ratio to find n(Na₂CO₃) = 0.20 ÷ 2 = 0.10 mol.
    • Calculates Mr(Na₂CO₃) = (2×23) + 12 + (3×16) = 106.
    • Uses mass = moles × Mr.
    • Obtains mass = 0.10 × 106 = 10.6 g.

    Examiner tip: Always convert to moles first using the balanced equation ratio before converting back to mass with the relative formula mass.

  10. 10.

    Explain why the actual (experimental) yield of a chemical reaction is usually less than the theoretical yield calculated from the balanced equation.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that some product may be lost during practical steps such as filtration, transfer between containers, or purification. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that some reactants may undergo side reactions, producing different (unwanted) products instead of the main product. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that some reactions do not go to completion (are reversible or incomplete), leaving unreacted starting material. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Percentage yield = (actual yield ÷ theoretical yield) × 100 is always less than or equal to 100%, reflecting practical losses at every stage of a real experiment.

    Marking points

    • States that some product may be lost during practical steps such as filtration, transfer between containers, or purification.
    • States that some reactants may undergo side reactions, producing different (unwanted) products instead of the main product.
    • States that some reactions do not go to completion (are reversible or incomplete), leaving unreacted starting material.

    Examiner tip: Percentage yield = (actual yield ÷ theoretical yield) × 100 is always less than or equal to 100%, reflecting practical losses at every stage of a real experiment.

  11. 11.

    25.0 cm³ of sodium hydroxide solution is exactly neutralised by 20.0 cm³ of 0.15 mol/dm³ hydrochloric acid: NaOH + HCl → NaCl + H₂O. Calculate the concentration of the sodium hydroxide solution.

    [3 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Work through this mathematical step: Calculates moles of HCl = 0.15 × 0.0200 = 0.0030 mol. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Work through this mathematical step: Uses the 1:1 equation ratio to state moles of NaOH = 0.0030 mol. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Work through this mathematical step: Uses concentration = moles ÷ volume (dm³) = 0.0030 ÷ 0.0250 to obtain 0.12 mol/dm³. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Always convert titration volumes from cm³ to dm³ (divide by 1000) before using them in concentration calculations.

    Marking points

    • Calculates moles of HCl = 0.15 × 0.0200 = 0.0030 mol.
    • Uses the 1:1 equation ratio to state moles of NaOH = 0.0030 mol.
    • Uses concentration = moles ÷ volume (dm³) = 0.0030 ÷ 0.0250 to obtain 0.12 mol/dm³.

    Examiner tip: Always convert titration volumes from cm³ to dm³ (divide by 1000) before using them in concentration calculations.

  12. 12.

    A 2.50 g sample of hydrated copper(II) sulfate, CuSO₄·xH₂O, is heated to constant mass, leaving 1.60 g of anhydrous copper(II) sulfate. Calculate the value of x. Use Mr: CuSO₄ = 160, H₂O = 18.

    [4 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Work through this mathematical step: Calculates moles of anhydrous CuSO₄ = 1.60 ÷ 160 = 0.0100 mol. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Work through this mathematical step: Calculates mass of water lost = 2.50 − 1.60 = 0.90 g, and moles of water = 0.90 ÷ 18 = 0.050 mol. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Work through this mathematical step: Calculates the ratio of moles of water to moles of CuSO₄ = 0.050 ÷ 0.0100 = 5. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    5. Work through this mathematical step: States x = 5 (the compound is CuSO₄·5H₂O). Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Water of crystallisation calculations always compare the moles of water lost to the moles of the anhydrous salt remaining, never their masses directly.

    Marking points

    • Calculates moles of anhydrous CuSO₄ = 1.60 ÷ 160 = 0.0100 mol.
    • Calculates mass of water lost = 2.50 − 1.60 = 0.90 g, and moles of water = 0.90 ÷ 18 = 0.050 mol.
    • Calculates the ratio of moles of water to moles of CuSO₄ = 0.050 ÷ 0.0100 = 5.
    • States x = 5 (the compound is CuSO₄·5H₂O).

    Examiner tip: Water of crystallisation calculations always compare the moles of water lost to the moles of the anhydrous salt remaining, never their masses directly.

  13. 13.

    Calcium carbonate decomposes on heating: CaCO₃ → CaO + CO₂. Calculate the mass of calcium oxide produced from 25.0 g of calcium carbonate. Use Mr: CaCO₃ = 100, CaO = 56.

    [3 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Work through this mathematical step: Calculates moles of CaCO₃ = 25.0 ÷ 100 = 0.25 mol. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Work through this mathematical step: Uses the 1:1 equation ratio to state moles of CaO = 0.25 mol. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Work through this mathematical step: Obtains mass of CaO = 0.25 × 56 = 14 g. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Reacting-mass problems always follow the same route: mass to moles using the given substance's Mr, apply the equation ratio, then moles back to mass using the target substance's Mr.

    Marking points

    • Calculates moles of CaCO₃ = 25.0 ÷ 100 = 0.25 mol.
    • Uses the 1:1 equation ratio to state moles of CaO = 0.25 mol.
    • Obtains mass of CaO = 0.25 × 56 = 14 g.

    Examiner tip: Reacting-mass problems always follow the same route: mass to moles using the given substance's Mr, apply the equation ratio, then moles back to mass using the target substance's Mr.

  14. 14.

    Zinc reacts with excess hydrochloric acid: Zn + 2HCl → ZnCl₂ + H₂. Calculate the volume of hydrogen gas produced at room temperature and pressure (rtp) from 6.5 g of zinc. Use Ar: Zn = 65; 1 mole of gas occupies 24 dm³ at rtp.

    [4 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Work through this mathematical step: Calculates moles of Zn = 6.5 ÷ 65 = 0.10 mol. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Work through this mathematical step: Uses the 1:1 equation ratio to state moles of H₂ = 0.10 mol. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Work through this mathematical step: Uses volume = moles × 24 dm³. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    5. Work through this mathematical step: Obtains volume of H₂ = 2.4 dm³. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Since zinc is a solid and hydrochloric acid is in excess, zinc's mass is what limits the reaction — always identify the limiting reactant before starting a gas-volume calculation.

    Marking points

    • Calculates moles of Zn = 6.5 ÷ 65 = 0.10 mol.
    • Uses the 1:1 equation ratio to state moles of H₂ = 0.10 mol.
    • Uses volume = moles × 24 dm³.
    • Obtains volume of H₂ = 2.4 dm³.

    Examiner tip: Since zinc is a solid and hydrochloric acid is in excess, zinc's mass is what limits the reaction — always identify the limiting reactant before starting a gas-volume calculation.

  15. 15.

    A solution of sodium hydroxide has a concentration of 8.0 g/dm³. Calculate its concentration in mol/dm³. Use Mr: NaOH = 40.

    [2 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Work through this mathematical step: Uses concentration (mol/dm³) = concentration (g/dm³) ÷ Mr. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Work through this mathematical step: Obtains concentration = 8.0 ÷ 40 = 0.20 mol/dm³. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Concentration can be expressed in either g/dm³ or mol/dm³ — converting between them always requires dividing or multiplying by the solute's relative formula mass.

    Marking points

    • Uses concentration (mol/dm³) = concentration (g/dm³) ÷ Mr.
    • Obtains concentration = 8.0 ÷ 40 = 0.20 mol/dm³.

    Examiner tip: Concentration can be expressed in either g/dm³ or mol/dm³ — converting between them always requires dividing or multiplying by the solute's relative formula mass.

  16. 16.

    0.20 mol of magnesium is reacted with 0.30 mol of hydrochloric acid: Mg + 2HCl → MgCl₂ + H₂. Determine which reactant is in excess, and calculate how many moles of it remain unreacted.

    [4 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Work through this mathematical step: Calculates that 0.20 mol Mg would require 0.20 × 2 = 0.40 mol HCl to react completely. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Develop this part of the answer: States that since only 0.30 mol HCl is available (less than 0.40 mol), HCl is the limiting reactant and Mg is in excess. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Work through this mathematical step: Calculates the moles of Mg that react with the available HCl: 0.30 ÷ 2 = 0.15 mol. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    5. Work through this mathematical step: Obtains moles of Mg remaining unreacted = 0.20 − 0.15 = 0.05 mol. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: To find the limiting reactant, always calculate how much of one reactant would be needed to react completely with the other, then compare this to the amount actually available.

    Marking points

    • Calculates that 0.20 mol Mg would require 0.20 × 2 = 0.40 mol HCl to react completely.
    • States that since only 0.30 mol HCl is available (less than 0.40 mol), HCl is the limiting reactant and Mg is in excess.
    • Calculates the moles of Mg that react with the available HCl: 0.30 ÷ 2 = 0.15 mol.
    • Obtains moles of Mg remaining unreacted = 0.20 − 0.15 = 0.05 mol.

    Examiner tip: To find the limiting reactant, always calculate how much of one reactant would be needed to react completely with the other, then compare this to the amount actually available.

  17. 17.

    25.0 cm³ of 0.20 mol/dm³ sodium hydroxide solution is exactly neutralised by 15.0 cm³ of sulfuric acid: H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O. Calculate the concentration of the sulfuric acid.

    [4 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Work through this mathematical step: Calculates moles of NaOH = 0.20 × 0.0250 = 0.0050 mol. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Work through this mathematical step: Uses the 2:1 equation ratio to state moles of H₂SO₄ = 0.0050 ÷ 2 = 0.0025 mol. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Work through this mathematical step: Uses concentration = moles ÷ volume (dm³) = 0.0025 ÷ 0.0150. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    5. Develop this part of the answer: Obtains concentration ≈ 0.17 mol/dm³. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: When an acid and base react in a ratio other than 1:1, always divide by the correct equation coefficient before finding the second substance's concentration.

    Marking points

    • Calculates moles of NaOH = 0.20 × 0.0250 = 0.0050 mol.
    • Uses the 2:1 equation ratio to state moles of H₂SO₄ = 0.0050 ÷ 2 = 0.0025 mol.
    • Uses concentration = moles ÷ volume (dm³) = 0.0025 ÷ 0.0150.
    • Obtains concentration ≈ 0.17 mol/dm³.

    Examiner tip: When an acid and base react in a ratio other than 1:1, always divide by the correct equation coefficient before finding the second substance's concentration.

  18. 18.

    A student calculates a theoretical yield of 12.0 g for a preparation, but obtains an actual yield of 9.6 g. Calculate the percentage yield.

    [2 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Work through this mathematical step: Uses percentage yield = (actual yield ÷ theoretical yield) × 100. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Work through this mathematical step: Obtains percentage yield = (9.6 ÷ 12.0) × 100 = 80%. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Percentage yield can never exceed 100% for a correctly balanced equation and accurate measurements, since the actual yield cannot exceed the theoretical maximum.

    Marking points

    • Uses percentage yield = (actual yield ÷ theoretical yield) × 100.
    • Obtains percentage yield = (9.6 ÷ 12.0) × 100 = 80%.

    Examiner tip: Percentage yield can never exceed 100% for a correctly balanced equation and accurate measurements, since the actual yield cannot exceed the theoretical maximum.

  19. 19.

    2 dm³ of hydrogen gas react completely with 1 dm³ of oxygen gas to form water vapour: 2H₂ + O₂ → 2H₂O. State the law that explains why the ratio of reacting gas volumes matches the ratio of moles in the equation, and calculate the volume of oxygen needed to react completely with 10 dm³ of hydrogen.

    [3 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Develop this part of the answer: States Avogadro's law: equal volumes of different gases, at the same temperature and pressure, contain equal numbers of molecules (moles). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Work through this mathematical step: States that this means the ratio of reacting gas volumes equals the ratio of moles in the balanced equation. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Work through this mathematical step: Uses the 2:1 ratio of hydrogen to oxygen to obtain volume of oxygen = 10 ÷ 2 = 5 dm³. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: For reactions between gases, volumes can be used directly in the same ratio as the equation's coefficients, without ever converting to moles first.

    Marking points

    • States Avogadro's law: equal volumes of different gases, at the same temperature and pressure, contain equal numbers of molecules (moles).
    • States that this means the ratio of reacting gas volumes equals the ratio of moles in the balanced equation.
    • Uses the 2:1 ratio of hydrogen to oxygen to obtain volume of oxygen = 10 ÷ 2 = 5 dm³.

    Examiner tip: For reactions between gases, volumes can be used directly in the same ratio as the equation's coefficients, without ever converting to moles first.

  20. 20.

    Calculate the number of moles of solute in 250 cm³ of a 0.40 mol/dm³ solution of potassium nitrate.

    [2 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Work through this mathematical step: Uses moles = concentration × volume (dm³). Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Work through this mathematical step: Obtains moles = 0.40 × 0.250 = 0.10 mol. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Volume must be in dm³ (not cm³) when using this formula — divide a volume in cm³ by 1000 first.

    Marking points

    • Uses moles = concentration × volume (dm³).
    • Obtains moles = 0.40 × 0.250 = 0.10 mol.

    Examiner tip: Volume must be in dm³ (not cm³) when using this formula — divide a volume in cm³ by 1000 first.

  21. 21.

    Calculate the mass of sodium hydroxide needed to prepare 500 cm³ of a 0.20 mol/dm³ solution. Use Mr: NaOH = 40.

    [3 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Work through this mathematical step: Calculates moles needed = concentration × volume (dm³) = 0.20 × 0.500 = 0.10 mol. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Work through this mathematical step: Uses mass = moles × Mr. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Work through this mathematical step: Obtains mass = 0.10 × 40 = 4.0 g. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Preparing a solution of known concentration always works backwards from the target moles (concentration × volume) to the mass needed.

    Marking points

    • Calculates moles needed = concentration × volume (dm³) = 0.20 × 0.500 = 0.10 mol.
    • Uses mass = moles × Mr.
    • Obtains mass = 0.10 × 40 = 4.0 g.

    Examiner tip: Preparing a solution of known concentration always works backwards from the target moles (concentration × volume) to the mass needed.

  22. 22.

    180 cm³ of oxygen gas at room temperature and pressure (rtp) reacts completely with excess magnesium: 2Mg + O₂ → 2MgO. Calculate the mass of magnesium oxide formed. Use Ar: Mg = 24, O = 16; 1 mole of gas occupies 24 dm³ at rtp.

    [5 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Work through this mathematical step: Converts the gas volume to dm³: 180 cm³ = 0.180 dm³. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Work through this mathematical step: Calculates moles of O₂ = 0.180 ÷ 24 = 0.0075 mol. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Work through this mathematical step: Uses the 1:2 equation ratio to state moles of MgO = 0.0075 × 2 = 0.015 mol. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    5. Work through this mathematical step: Calculates Mr(MgO) = 24 + 16 = 40. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    6. Work through this mathematical step: Obtains mass of MgO = 0.015 × 40 = 0.60 g. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    7. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Since magnesium is in excess, the volume of oxygen gas is what limits how much product forms — work from the limiting reactant's amount throughout.

    Marking points

    • Converts the gas volume to dm³: 180 cm³ = 0.180 dm³.
    • Calculates moles of O₂ = 0.180 ÷ 24 = 0.0075 mol.
    • Uses the 1:2 equation ratio to state moles of MgO = 0.0075 × 2 = 0.015 mol.
    • Calculates Mr(MgO) = 24 + 16 = 40.
    • Obtains mass of MgO = 0.015 × 40 = 0.60 g.

    Examiner tip: Since magnesium is in excess, the volume of oxygen gas is what limits how much product forms — work from the limiting reactant's amount throughout.

  23. 23.

    A 0.50 mol sample of a compound has a mass of 45 g. Calculate its relative formula mass (Mr).

    [2 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Work through this mathematical step: Uses Mr = mass ÷ moles. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Work through this mathematical step: Obtains Mr = 45 ÷ 0.50 = 90. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: This is simply the amount-of-substance formula rearranged: moles = mass ÷ Mr becomes Mr = mass ÷ moles when Mr is the unknown.

    Marking points

    • Uses Mr = mass ÷ moles.
    • Obtains Mr = 45 ÷ 0.50 = 90.

    Examiner tip: This is simply the amount-of-substance formula rearranged: moles = mass ÷ Mr becomes Mr = mass ÷ moles when Mr is the unknown.

  24. 24.

    Calculate the volume of 0.50 mol/dm³ hydrochloric acid required to react exactly with 2.65 g of sodium carbonate: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. Use Mr: Na₂CO₃ = 106.

    [4 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Work through this mathematical step: Calculates moles of Na₂CO₃ = 2.65 ÷ 106 = 0.025 mol. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Work through this mathematical step: Uses the 1:2 equation ratio to state moles of HCl = 0.025 × 2 = 0.050 mol. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Work through this mathematical step: Uses volume = moles ÷ concentration. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    5. Work through this mathematical step: Obtains volume = 0.050 ÷ 0.50 = 0.100 dm³ (100 cm³). Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: This calculation runs mass → moles → moles (via the equation ratio) → volume, the reverse of a standard titration concentration calculation.

    Marking points

    • Calculates moles of Na₂CO₃ = 2.65 ÷ 106 = 0.025 mol.
    • Uses the 1:2 equation ratio to state moles of HCl = 0.025 × 2 = 0.050 mol.
    • Uses volume = moles ÷ concentration.
    • Obtains volume = 0.050 ÷ 0.50 = 0.100 dm³ (100 cm³).

    Examiner tip: This calculation runs mass → moles → moles (via the equation ratio) → volume, the reverse of a standard titration concentration calculation.

  25. 25.

    Calculate the percentage by mass of water in hydrated magnesium sulfate, MgSO₄·7H₂O. Use Ar: Mg = 24, S = 32, O = 16, H = 1.

    [4 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Work through this mathematical step: Calculates Mr(MgSO₄) = 24 + 32 + (4×16) = 120. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Work through this mathematical step: Calculates the mass of 7H₂O = 7 × 18 = 126. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Work through this mathematical step: States Mr(MgSO₄·7H₂O) = 120 + 126 = 246. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    5. Work through this mathematical step: Obtains percentage by mass of water = (126 ÷ 246) × 100 ≈ 51.2%. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Hydrated salts like Epsom salt (MgSO₄·7H₂O) can lose more than half their mass as water on heating — this is why water of crystallisation calculations often give surprisingly large percentages.

    Marking points

    • Calculates Mr(MgSO₄) = 24 + 32 + (4×16) = 120.
    • Calculates the mass of 7H₂O = 7 × 18 = 126.
    • States Mr(MgSO₄·7H₂O) = 120 + 126 = 246.
    • Obtains percentage by mass of water = (126 ÷ 246) × 100 ≈ 51.2%.

    Examiner tip: Hydrated salts like Epsom salt (MgSO₄·7H₂O) can lose more than half their mass as water on heating — this is why water of crystallisation calculations often give surprisingly large percentages.

Original, unofficial practice questions from SubjectScout — not official past-paper questions.