# Explain why a buffer solution formed by mixing excess ethanoic acid with sodium hydroxide solution resists a large change in pH when a small amount of strong base is added.

> IB DP Chemistry Higher Level (2023 syllabus) — R3.1 Proton transfer reactions · Explain · 4 marks

> A buffer solution contains significant concentrations of both ethanoic acid (CH₃COOH) and sodium ethanoate (CH₃COO⁻Na⁺) in equilibrium: CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq).

## Mark scheme (4 marks)

1. The buffer contains a reservoir of the weak acid (CH₃COOH) and its conjugate base (CH₃COO⁻) in significant concentrations.
2. When strong base (OH⁻) is added, it reacts with the ethanoic acid: CH₃COOH + OH⁻ → CH₃COO⁻ + H₂O.
3. This reaction removes most of the added OH⁻, preventing a large increase in [OH⁻] / preventing a large decrease in [H⁺].
4. The ratio [CH₃COO⁻]/[CH₃COOH] changes only slightly, so by the Henderson–Hasselbalch equation (pH = pKa + log([A⁻]/[HA])), the pH changes only slightly.

## Key terms

- [buffer solution](https://www.gradenine.co.uk/glossary/buffer-solution)

## Related

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