# A student measures the pH of two separate solutions: a 0.1 mol/dm³ solution of nitric acid and a 0.1 mol/dm³ solution of methanoic acid. The nitric acid has a lower pH than the methanoic acid. Explain why the two solutions have different pH values, even though they have the same concentration.

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

> Nitric acid is a strong acid. Methanoic acid is a weak acid.

## Mark scheme (5 marks)

1. Strong acids fully/completely ionise (in solution)
2. Weak acids only partially ionise (in solution)
3. Nitric acid produces a higher concentration of hydrogen ions (H⁺) than methanoic acid at the same concentration
4. Lower concentration of H⁺ ions (from methanoic acid) gives a higher pH
5. pH is a measure of hydrogen ion concentration, so a higher [H⁺] gives a lower 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-measures-the-ph-of-a389f9af) · Published by Druglandscape Ltd.