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A-Level Chemistry October/November 2024 Q1(a)(iii): Write two equations to show how a mixture of (Na+)2(HPO42-) and Na⁺(H2PO4¯) can act as…
A-Level Chemistry · Paper 9701/41 · October/November 2024 · Question 1(a)(iii) · [2 marks]
Write two equations to show how a mixture of (Na+)2(HPO42-) and Na⁺(H2PO4¯) can act as a buffer solution. equation 1 equation 2
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
- accepted ✓
A buffer solution resists changes in pH upon the addition of small amounts of acid or alkali.
Equation 1: Reaction with added acid (H⁺)
The hydrogenphosphate ion (the conjugate base) removes added H⁺ ions.
HPO₄²⁻(aq) + H⁺(aq) → H₂PO₄⁻(aq)
Equation 2: Reaction with added alkali (OH⁻)
The dihydrogenphosphate ion (the weak acid) removes added OH⁻ ions.
H₂PO₄⁻(aq) + OH⁻(aq) → HPO₄²⁻(aq) + H₂O(l)
How the marks are awarded
- B1 — The first mark is for the correct equation showing the conjugate base, HPO₄²⁻, reacting with an acid/H⁺ to form the weak acid, H₂PO₄⁻.
- B1 — The second mark is for the correct equation showing the weak acid, H₂PO₄⁻, reacting with a base/OH⁻ to form the conjugate base, HPO₄²⁻, and water.
Common mistakes
- Reversing the roles of the acid and base components, for example, showing H₂PO₄⁻ reacting with H⁺.
- Using equilibrium arrows (⇌) instead of a forward arrow (→). The reaction to remove added acid or alkali goes to completion.
- Incorrectly balancing charges, for example, writing HPO₄⁻ instead of HPO₄²⁻, or omitting the charges altogether.
- Forgetting to include water (H₂O) as a product in the reaction with hydroxide ions (OH⁻).
Examiner tip: For any buffer system, identify the weak acid and conjugate base components, then write the equation for the acid part neutralising added alkali (OH⁻) and the base part neutralising added acid (H⁺).
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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