# A buffer solution can be made by mixing propanoic acid (CH₃CH₂COOH) with sodium propanoate (CH₃CH₂COONa). 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

> Propanoic acid is a weak acid with Ka = 1.34 × 10⁻⁵ mol dm⁻³. When sodium propanoate dissolves in water it fully dissociates to give propanoate ions (CH₃CH₂COO⁻) and sodium ions.

## Mark scheme (5 marks)

1. The buffer contains a high concentration of propanoate ions (CH₃CH₂COO⁻), which act as the conjugate base / reservoir of base
2. The added H⁺ ions (from HCl) react with the propanoate ions
3. This produces propanoic acid (CH₃CH₂COOH), converting the added H⁺ into a weak acid
4. Because propanoic acid is a weak acid, it only partially dissociates, so the concentration of H⁺ ions in solution does not significantly increase
5. Therefore the pH remains approximately constant / changes only very slightly

## Key terms

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

## 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)
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