# A student tests two solutions, A and B, both at the same concentration. Solution A is sodium hydroxide and solution B is ammonia solution. Both solutions have a pH above 7, but solution A has a higher pH than solution B. Explain why solution A has a higher pH than solution B, even though both solutions are at the same concentration.

> WJEC A-Level Chemistry (Wales) — 3.9 Acid-base equilibria · Explain · 5 marks

## Mark scheme (5 marks)

1. Sodium hydroxide is a strong base / alkali
2. Sodium hydroxide completely / fully dissociates (ionises) in water
3. Ammonia is a weak base / alkali
4. Ammonia only partially dissociates (ionises) in water / an equilibrium is established
5. Therefore solution A has a higher concentration of OH⁻ ions than solution B (at the same concentration), giving it a higher pH

## Key terms

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

## Related

- [Revision notes for WJEC A-Level Chemistry (Wales)](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-tests-two-solutions-a-7481debe) · Published by Druglandscape Ltd.