# A student adds a small volume of dilute hydrochloric acid to a buffer solution made from methanoic acid (HCOOH) and sodium methanoate (HCOONa). Explain why the pH of the buffer solution remains approximately constant.

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

## Mark scheme (5 marks)

1. The buffer contains a large reservoir of methanoate ions (HCOO⁻) from the sodium methanoate
2. The added H⁺ ions react with / are removed by the methanoate ions
3. This forms methanoic acid (HCOOH), so H⁺ ions are not allowed to accumulate
4. There is also a large reservoir of undissociated methanoic acid (HCOOH) present
5. Because the change in [H⁺] is very small, the pH remains approximately constant

## 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-student-adds-a-small-volume-586ed672) · Published by Druglandscape Ltd.