# A student measures the pH of two solutions: a 0.1 mol/dm³ solution of sodium hydroxide and a 0.1 mol/dm³ solution of ammonia. The sodium hydroxide solution has a pH of 13, while the ammonia solution has a pH of 11. Explain why the two solutions have different pH values, even though they have the same concentration.

> Eduqas A-Level Chemistry — 3.9 Acid-base equilibria · Explain · 5 marks

> Sodium hydroxide is classified as a strong base. Ammonia is classified as a weak base.

## Mark scheme (5 marks)

1. Sodium hydroxide is fully/completely dissociated (in aqueous solution)
2. Ammonia is only partially dissociated / partially ionised (in aqueous solution)
3. This means the concentration of hydroxide ions (OH⁻) is higher in the sodium hydroxide solution than in the ammonia solution
4. A higher concentration of OH⁻ ions results in a higher pH
5. The equilibrium in the ammonia solution lies to the left / favours the undissociated form, so fewer OH⁻ ions are produced

## Key terms

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

## Related

- [Revision notes for Eduqas A-Level Chemistry](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-measures-the-ph-of-ba9f62a5) · Published by Druglandscape Ltd.