# A buffer solution is prepared by dissolving sodium ethanoate in excess ethanoic acid. Explain how this buffer solution resists a change in pH when a small amount of hydrochloric acid is added to it.

> Edexcel A-Level Chemistry (9CH0) — 11.1 pH, Ka and buffer solutions · Explain · 5 marks

> Ethanoic acid is a weak acid that partially dissociates in aqueous solution. Its conjugate base, the ethanoate ion, is present in relatively high concentration in the buffer solution.

## Mark scheme (5 marks)

1. Hydrochloric acid is a source of hydrogen ions (H⁺ / H₃O⁺)
2. The ethanoate ions (CH₃COO⁻) in the buffer react with / remove the added H⁺ ions
3. This reaction produces more ethanoic acid (CH₃COOH), reforming the weak acid
4. The concentration of H⁺ ions in solution does not change significantly / remains approximately constant
5. Therefore the pH remains approximately constant / does not change significantly

## Key terms

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

## Related

- [Revision notes for Edexcel A-Level Chemistry (9CH0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-buffer-solution-is-prepared-by-1f2b5c92) · Published by Druglandscape Ltd.