Chemistry HL
Reactivity 2: acid-base equilibria (HL) — Reactivity 2 HL
- 1.
At 298 K, calculate the pH of a 0.0200 mol dm⁻³ solution of the strong base NaOH, assuming complete dissociation.
[3 marks] - 2.
Marking analysis: A learner attempts the following task: “At 298 K, calculate the pH of a 0.0200 mol dm⁻³ solution of the strong base NaOH, assuming complete dissociation.” Their response addresses only this point: “States that [OH⁻] = 0.0200 mol dm⁻³ and calculates pOH = −log₁₀(0.0200) ≈ 1.70.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[3 marks] - 3.
Ethanoic acid (CH₃COOH) has Ka = 1.8 × 10⁻⁵. Calculate the pH of a 0.100 mol dm⁻³ solution of ethanoic acid, stating one assumption made.
[4 marks] - 4.
Marking analysis: A learner attempts the following task: “Ethanoic acid (CH₃COOH) has Ka = 1.8 × 10⁻⁵. Calculate the pH of a 0.100 mol dm⁻³ solution of ethanoic acid, stating one assumption made.” Their response addresses only this point: “Sets up Ka = [H⁺][A⁻]/[HA] ≈ [H⁺]²/[HA]₀, assuming [H⁺] is small compared to the initial acid concentration so [HA] ≈ 0.100.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[4 marks] - 5.
Define a buffer solution, and explain how a mixture of ethanoic acid and sodium ethanoate resists changes in pH when a small amount of acid is added.
[4 marks] · no calculator - 6.
Marking analysis: A learner attempts the following task: “Define a buffer solution, and explain how a mixture of ethanoic acid and sodium ethanoate resists changes in pH when a small amount of acid is added.” Their response addresses only this point: “Defines a buffer solution as one that resists changes in pH when small amounts of acid or base are added.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[4 marks] · no calculator - 7.
A buffer solution is prepared containing 0.200 mol dm⁻³ ethanoic acid (Ka = 1.8 × 10⁻⁵) and 0.150 mol dm⁻³ sodium ethanoate. Calculate the pH of this buffer using the Henderson-Hasselbalch equation.
[4 marks] - 8.
Marking analysis: A learner attempts the following task: “A buffer solution is prepared containing 0.200 mol dm⁻³ ethanoic acid (Ka = 1.8 × 10⁻⁵) and 0.150 mol dm⁻³ sodium ethanoate. Calculate the pH of this buffer using the Henderson-Hasselbalch equation.” Their response addresses only this point: “States the Henderson-Hasselbalch equation: pH = pKa + log₁₀([A⁻]/[HA]).” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[4 marks] - 9.
Sketch, in words, the shape of the pH curve produced when a strong base is added gradually to a strong acid, and state the approximate pH at the equivalence point.
[3 marks] · no calculator - 10.
Marking analysis: A learner attempts the following task: “Sketch, in words, the shape of the pH curve produced when a strong base is added gradually to a strong acid, and state the approximate pH at the equivalence point.” Their response addresses only this point: “States that the pH starts low and rises slowly at first as base is added.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[3 marks] · no calculator - 11.
Explain why the equivalence point for the titration of a weak acid with a strong base occurs above pH 7.
[3 marks] · no calculator - 12.
Marking analysis: A learner attempts the following task: “Explain why the equivalence point for the titration of a weak acid with a strong base occurs above pH 7.” Their response addresses only this point: “States that at the equivalence point, the weak acid has been fully converted to its conjugate base (a salt) in solution.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[3 marks] · no calculator - 13.
State the criterion for choosing a suitable indicator for an acid-base titration, in terms of the indicator's pH range and the pH curve.
[2 marks] · no calculator - 14.
Marking analysis: A learner attempts the following task: “State the criterion for choosing a suitable indicator for an acid-base titration, in terms of the indicator's pH range and the pH curve.” Their response addresses only this point: “States that a suitable indicator is one whose colour-change pH range falls entirely within the steep, near-vertical portion of the titration's pH curve.” Evaluate the response against the complete 2-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[2 marks] · no calculator - 15.
A buffer solution has a pH of 5.20. A small amount of concentrated HCl is added. Explain, referring to Le Chatelier's principle, why the pH of the buffer changes only slightly rather than dropping sharply.
[3 marks] · no calculator - 16.
Marking analysis: A learner attempts the following task: “A buffer solution has a pH of 5.20. A small amount of concentrated HCl is added. Explain, referring to Le Chatelier's principle, why the pH of the buffer changes only slightly rather than dropping sharply.” Their response addresses only this point: “States that the added H⁺ increases the concentration of H⁺ in the equilibrium HA ⇌ H⁺ + A⁻.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[3 marks] · no calculator - 17.
Calculate the pOH and hence the pH of a 0.0250 mol dm⁻³ solution of the strong base NaOH at 298 K, given Kw = 1.0 × 10⁻¹⁴.
[4 marks] - 18.
Marking analysis: A learner attempts the following task: “Calculate the pOH and hence the pH of a 0.0250 mol dm⁻³ solution of the strong base NaOH at 298 K, given Kw = 1.0 × 10⁻¹⁴.” Their response addresses only this point: “States that for a strong monobasic base, [OH⁻] equals the base concentration: [OH⁻] = 0.0250 mol dm⁻³.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[4 marks] - 19.
Outline why a buffer solution has a limited buffering capacity, referring to what happens if a large excess of strong acid is added.
[2 marks] · no calculator - 20.
Marking analysis: A learner attempts the following task: “Outline why a buffer solution has a limited buffering capacity, referring to what happens if a large excess of strong acid is added.” Their response addresses only this point: “States that the buffer's capacity to neutralize added acid depends on the finite amount of conjugate base (A⁻) present.” Evaluate the response against the complete 2-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[2 marks] · no calculator - 21.
Ammonia has Kb = 1.8 × 10⁻⁵. Calculate the pH of a 0.150 mol dm⁻³ solution of ammonia at 298 K, stating one assumption made.
[5 marks] - 22.
Marking analysis: A learner attempts the following task: “Ammonia has Kb = 1.8 × 10⁻⁵. Calculate the pH of a 0.150 mol dm⁻³ solution of ammonia at 298 K, stating one assumption made.” Their response addresses only this point: “Sets up Kb = [NH₄⁺][OH⁻]/[NH₃] ≈ [OH⁻]²/[NH₃]₀, assuming [OH⁻] is small compared to the initial base concentration.” Evaluate the response against the complete 5-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[5 marks] - 23.
State the relationship between Ka of a weak acid and Kb of its conjugate base, and use this relationship to calculate Kb for the ethanoate ion, CH₃COO⁻, given Ka(CH₃COOH) = 1.8 × 10⁻⁵ and Kw = 1.0 × 10⁻¹⁴.
[3 marks] - 24.
Marking analysis: A learner attempts the following task: “State the relationship between Ka of a weak acid and Kb of its conjugate base, and use this relationship to calculate Kb for the ethanoate ion, CH₃COO⁻, given Ka(CH₃COOH) = 1.8 × 10⁻⁵ and Kw = 1.0 × 10⁻¹⁴.” Their response addresses only this point: “States that Ka × Kb = Kw for a conjugate acid-base pair.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[3 marks] - 25.
Phosphoric acid, H₃PO₄, is triprotic and has three successive acid dissociation constants, Ka1, Ka2 and Ka3. State how the relative sizes of Ka1, Ka2 and Ka3 compare, and explain why.
[3 marks] · no calculator - 26.
Marking analysis: A learner attempts the following task: “Phosphoric acid, H₃PO₄, is triprotic and has three successive acid dissociation constants, Ka1, Ka2 and Ka3. State how the relative sizes of Ka1, Ka2 and Ka3 compare, and explain why.” Their response addresses only this point: “States that Ka1 > Ka2 > Ka3, i.e. each successive dissociation constant is smaller than the last.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[3 marks] · no calculator - 27.
State two different methods that could be used to prepare a buffer solution containing ethanoic acid and ethanoate ions.
[2 marks] · no calculator - 28.
Marking analysis: A learner attempts the following task: “State two different methods that could be used to prepare a buffer solution containing ethanoic acid and ethanoate ions.” Their response addresses only this point: “States that a solution of a weak acid (e.g. ethanoic acid) can be mixed directly with a solution of a soluble salt of its conjugate base (e.g. sodium ethanoate).” Evaluate the response against the complete 2-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[2 marks] · no calculator - 29.
A 0.200 dm³ buffer initially contains 0.0300 mol CH₃COOH and 0.0200 mol CH₃COO⁻. A student adds 0.00500 mol HCl and assumes the volume change is negligible. The pKa of ethanoic acid is 4.76. (a) Write the reaction of added H⁺ with the buffer base. (b) Calculate the moles of acid and base after reaction. (c) Calculate the new pH. (d) Explain why the pH change is limited.
[5 marks] - 30.
Marking analysis: A learner attempts the following task: “A 0.200 dm³ buffer initially contains 0.0300 mol CH₃COOH and 0.0200 mol CH₃COO⁻. A student adds 0.00500 mol HCl and assumes the volume change is negligible. The pKa of ethanoic acid is 4.76. (a) Write the reaction of added H⁺ with the buffer base. (b) Calculate the moles of acid and base after reaction. (c) Calculate the new pH. (d) Explain why the pH change is limited.” Their response addresses only this point: “Writes CH₃COO⁻ + H⁺ → CH₃COOH.” Evaluate the response against the complete 5-mark task. Identify what earns credit and state every additional requirement needed for full marks.
[5 marks]