Chemistry

1. 1.

   Calculate the amount in moles in 5.85 g of NaCl using Mr = 58.5.

   [2 marks]

   Answer explanation

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

   1. Divide the measured mass by the molar mass: 5.85/58.5 = 0.100 mol. The relative formula mass gives the numerical molar mass in g mol^-1.

   Marking points

   • Use n = m/M.
   • n = 0.100 mol.

   Examiner tip: Multiplying mass by molar mass gives the wrong dimensions.

2. 2.

   Explain why a burette is rinsed with the titrant before filling, rather than leaving distilled water inside.

   [3 marks] · no calculator

   Answer explanation

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

   1. Volume readings remain possible after dilution, but converting that volume into moles with the original concentration becomes invalid. Conditioning the burette prevents this systematic error.

   Marking points

   • Residual water dilutes the titrant.
   • Its actual concentration would differ from the stated concentration.
   • Rinsing with titrant displaces water without changing the intended composition.

   Examiner tip: Do not confuse the burette rinse with adding distilled water to the conical flask.

3. 3.

   25.0 cm^3 of H2SO4 is neutralised by 30.0 cm^3 of 0.200 mol dm^-3 NaOH. Calculate the acid concentration; H2SO4 + 2NaOH -> Na2SO4 + 2H2O.

   [3 marks]

   Answer explanation

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

   1. Convert both volumes to dm^3. Calculate base moles as 0.200 x 0.0300, halve for the acid stoichiometry, then divide by 0.0250.

   Marking points

   • NaOH amount = 0.00600 mol.
   • Acid amount = 0.00300 mol.
   • Acid concentration = 0.120 mol dm^-3.

   Examiner tip: The mole ratio is not the volume ratio unless concentrations happen to match.

4. 4.

   A 20.00 cm^3 titre comes from two burette readings, each with absolute uncertainty +/-0.05 cm^3. Calculate the worst-case percentage uncertainty in the titre.

   [3 marks]

   Answer explanation

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

   1. A titre is final minus initial reading. Opposing reading errors can add in magnitude, so use (0.05 + 0.05)/20.00 x 100 = 0.50%, not a quadrature rule.

   Marking points

   • Absolute uncertainties add for subtraction in the stated worst-case model.
   • Titre uncertainty = +/-0.10 cm^3.
   • Percentage uncertainty = 0.50%.

   Examiner tip: The requested model is worst-case, not independent random-error propagation.

5. 5.

   10.0 g CaCO3 (M = 100 g mol^-1) reacts with 0.150 mol HCl: CaCO3 + 2HCl -> CaCl2 + CO2 + H2O. Calculate the maximum CO2 volume at 24.0 dm^3 mol^-1 and identify the limiting reagent.

   [4 marks]

   Answer explanation

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

   1. Compare each reagent's amount divided by its coefficient. The acid supplies only 0.150/2 reaction-moles, smaller than 0.100, so multiply 0.0750 by the stated molar gas volume.

   Marking points

   • CaCO3 amount = 0.100 mol.
   • HCl limits because 0.200 mol would be needed for all the carbonate.
   • CO2 amount = 0.0750 mol.
   • Maximum gas volume = 1.80 dm^3.

   Examiner tip: Do not choose a limiting reagent by comparing raw mole counts.

6. 6.

   A carbonate sample contains an insoluble inert impurity. A student weighs the sample, reacts it with excess acid and measures collected CO2. Evaluate two reasons why a low gas yield does not establish the impurity percentage reliably.

   [4 marks] · no calculator

   Answer explanation

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

   1. The calculation assumes every mole produced is collected. Separate chemical conversion from collection efficiency: excess acid addresses reaction completion, but not gas recovery.

   Marking points

   • Gas leakage before or during collection reduces measured yield.
   • Some CO2 may dissolve in the collection liquid.
   • Both losses mimic a lower carbonate content, biasing impurity estimates upwards.
   • A leak-tested apparatus and validated gas-recovery method/independent assay are needed.

   Examiner tip: Specify the direction of each bias rather than merely saying 'experimental error'.

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