# A student prepares a buffer solution by dissolving excess ammonium chloride in aqueous ammonia. Explain how this buffer solution resists a change in pH when a small amount of hydrochloric acid is added to it, and when a small amount of sodium hydroxide is added to it.

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

> A buffer solution maintains a nearly constant pH when small amounts of acid or alkali are added to it. The ammonium/ammonia buffer contains the weak base ammonia (NH₃) and its conjugate acid, the ammonium ion (NH₄⁺), in equilibrium: NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq)

## Mark scheme (5 marks)

1. The buffer contains a reservoir of NH₄⁺ ions (from the ammonium chloride) and NH₃ molecules (from the aqueous ammonia)
2. When hydrochloric acid is added, the H⁺ ions are removed by reacting with NH₃: NH₃ + H⁺ → NH₄⁺
3. Because the NH₃ reservoir is large, the change in [H⁺] and therefore pH is negligible when acid is added
4. When sodium hydroxide is added, the OH⁻ ions react with NH₄⁺ ions: NH₄⁺ + OH⁻ → NH₃ + H₂O
5. Because the NH₄⁺ reservoir is large, the change in [OH⁻] and therefore pH is negligible when alkali is added

## 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)
- [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-student-prepares-a-buffer-solution-9a80f5c0) · Published by Druglandscape Ltd.