# Explain why the pH of a solution of ammonium chloride (NH₄Cl) is less than 7 at 25 °C, making reference to the relevant equilibrium and the relative strengths of the species involved.

> IB DP Chemistry Higher Level (2023 syllabus) — R3.1 Proton transfer reactions · Explain · 4 marks

## Mark scheme (4 marks)

1. NH₄Cl is a fully dissociated salt, so the relevant species in solution is the ammonium ion, NH₄⁺.
2. NH₄⁺ acts as a weak Brønsted–Lowry acid, donating a proton to water: NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺.
3. The equilibrium lies to the left but does produce some H₃O⁺, making the solution acidic (pH < 7).
4. NH₄⁺ is a stronger acid than water (or equivalently, NH₃ is a weaker base than water / OH⁻), so the conjugate base NH₃ cannot fully neutralise the H₃O⁺ produced, leaving the solution acidic.

## Key terms

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

## Related

- [Revision notes for IB DP Chemistry Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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