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A-Level Chemistry May/June 2024 Q3(f): Buffer solutions are used to regulate pH. Write two equations to describe how a solutio…
A-Level Chemistry · Paper 9701/41 · May/June 2024 · Question 3(f) · [2 marks]
Buffer solutions are used to regulate pH. Write two equations to describe how a solution containing HC2O4¯ ions acts as a buffer solution when small amounts of acid or alkali are added.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
- accepted ✓
To show how a solution containing hydrogenoxalate ions, HC₂O₄⁻, acts as a buffer, we need to write two equations: one for its reaction with added acid (H⁺) and one for its reaction with added alkali (OH⁻).
Reaction with added acid: The HC₂O₄⁻ ion acts as a base, accepting a proton from the added acid.
HC₂O₄⁻ + H⁺ → H₂C₂O₄
Reaction with added alkali: The HC₂O₄⁻ ion acts as an acid, donating a proton to the added alkali.
HC₂O₄⁻ + OH⁻ → C₂O₄²⁻ + H₂O
How the marks are awarded
- M1 — Awarded for the correct equation showing the hydrogenoxalate ion, HC₂O₄⁻, acting as a base and reacting with a proton, H⁺, to form its conjugate acid, oxalic acid, H₂C₂O₄.
- M2 — Awarded for the correct equation showing the hydrogenoxalate ion, HC₂O₄⁻, acting as an acid and reacting with a hydroxide ion, OH⁻, to form its conjugate base, the oxalate ion, C₂O₄²⁻, and water, H₂O.
Common mistakes
- Using equilibrium arrows (⇌) instead of forward arrows (→). The equations should show the removal of the added H⁺ or OH⁻.
- Incorrectly balancing charges, for example writing C₂O₄⁻ instead of C₂O₄²⁻ as the product after reacting with OH⁻.
- Forgetting to include water (H₂O) as a product in the reaction with hydroxide ions (OH⁻).
- Confusing the roles and writing the reverse reactions, or starting with the wrong species (e.g. H₂C₂O₄ reacting with OH⁻, which is a valid buffer reaction but doesn't answer the specific question about the HC₂O₄⁻ ion).
Examiner tip: For any buffer question, identify the conjugate acid-base pair and write one equation where the base component neutralises added acid, and another where the acid component neutralises added alkali.
AI-generated model answer, grounded in the official Cambridge mark scheme and reviewed by the MarkScheme team. Mark your own answer to this question →
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