# Explain why ammonia solution is classified as a weak base and describe how this affects the pH of a 0.1 mol/dm³ ammonia solution compared to a 0.1 mol/dm³ sodium hydroxide solution.

> AQA A-Level Chemistry (7405) — 3.1.12 Acids and bases (A-Level only) · Explain · 5 marks

## Mark scheme (5 marks)

1. A weak base is only partially ionised / dissociated in aqueous solution
2. Ammonia reacts with water to produce OH⁻ ions / hydroxide ions, but the equilibrium lies to the left
3. Sodium hydroxide is a strong base / fully ionised in solution, producing a higher concentration of OH⁻ ions than ammonia at the same concentration
4. A higher concentration of OH⁻ ions means a higher pH
5. Therefore the ammonia solution has a lower pH than the sodium hydroxide solution (of the same concentration)

## Key terms

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

## Related

- [Revision notes for AQA A-Level Chemistry (7405)](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/explain-why-ammonia-solution-is-classified-02dc8724) · Published by Druglandscape Ltd.