# Propanoic acid (CH₃CH₂COOH) is a weak acid. Nitric acid (HNO₃) is a strong acid. Both acids can be prepared as solutions with the same concentration. Explain why, at the same concentration, a solution of nitric acid has a lower pH than a solution of propanoic acid.

> Edexcel A-Level Chemistry (9CH0) — 12.1 Strong and weak acids · Explain · 5 marks

> Propanoic acid is a weak acid used as a food preservative. Nitric acid is a strong acid used in the manufacture of fertilisers. Both acids release hydrogen ions in aqueous solution.

## Mark scheme (5 marks)

1. Nitric acid is a strong acid and fully/completely dissociates (ionises) in aqueous solution
2. Propanoic acid is a weak acid and only partially/incompletely dissociates (ionises) in aqueous solution
3. At the same concentration, nitric acid produces a greater concentration of hydrogen ions (H⁺/H₃O⁺) than propanoic acid
4. A higher concentration of hydrogen ions corresponds to a lower pH
5. Propanoic acid establishes a reversible equilibrium/the dissociation is reversible, so not all molecules release H⁺ ions

## Key terms

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

## Related

- [Revision notes for Edexcel A-Level Chemistry (9CH0)](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/propanoic-acid-ch-ch-cooh-is-a-weak-094b139e) · Published by Druglandscape Ltd.