# Explain why a buffer solution containing ethanoic acid and sodium ethanoate resists a significant change in pH when a small amount of hydrochloric acid is added to it.

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

> A buffer solution is prepared by mixing ethanoic acid (CH₃COOH) and sodium ethanoate (CH₃COONa) in water.

## Mark scheme (4 marks)

1. The buffer contains a reservoir of the conjugate base, ethanoate ions (CH₃COO⁻), in significant concentration.
2. The added H⁺ ions (from HCl) are consumed / neutralised by the ethanoate ions acting as a Brønsted–Lowry base.
3. Because the H⁺ ions are removed / converted to undissociated ethanoic acid, the H⁺ concentration and therefore pH remains almost unchanged.
4. The system can continue to resist pH change as long as the ethanoate ion reservoir is not exhausted (buffer capacity is maintained).

## Key terms

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

## Related

- [Revision notes for IB DP Chemistry Standard 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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