# Explain why the addition of a small amount of hydrochloric acid to a buffer solution containing ethanoic acid and sodium ethanoate produces only a negligible change in pH, with reference to the relevant equilibrium and the species present.

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

## Mark scheme (4 marks)

1. The buffer contains a reservoir of ethanoate ions (CH₃COO⁻) as the conjugate base.
2. The added H⁺ ions are consumed by reaction with ethanoate ions: CH₃COO⁻ + H⁺ → CH₃COOH.
3. This shifts the equilibrium CH₃COOH ⇌ CH₃COO⁻ + H⁺ to the left, so [H⁺] / pH changes only slightly.
4. Because the concentrations of both CH₃COOH and CH₃COO⁻ remain large relative to the small amount of acid added, the ratio [CH₃COO⁻]/[CH₃COOH] and hence pH (via the Henderson–Hasselbalch relationship) changes only negligibly.

## Key terms

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

## Related

- [Revision notes for IB DP Chemistry Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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