# A student has two acid solutions: hydrochloric acid and carbonic acid. Both solutions have the same concentration of 0.1 mol/dm³. The student measures the pH of each solution and finds that hydrochloric acid has a lower pH than carbonic acid at the same concentration. 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

> Hydrochloric acid is a strong acid. Carbonic acid is a weak acid.

## Mark scheme (5 marks)

1. Hydrochloric acid is fully / completely dissociated (ionised) in water
2. Carbonic acid is only partially / incompletely dissociated (ionised) in water
3. Hydrochloric acid produces a higher concentration of hydrogen ions (H⁺) than carbonic acid at the same concentration
4. pH is lower when the concentration of hydrogen ions is higher
5. Therefore hydrochloric acid has a lower pH because it has a greater hydrogen ion concentration than carbonic acid at the same overall concentration

## 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-has-two-acid-solutions-58ae5865) · Published by Druglandscape Ltd.