# Explain why a solution of ammonium chloride (NH₄Cl) is acidic at 25 °C, with reference to the Brønsted–Lowry theory of acids and bases.

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

## Mark scheme (4 marks)

1. NH₄Cl fully dissociates in water to give NH₄⁺ and Cl⁻ ions
2. NH₄⁺ is a Brønsted–Lowry acid because it donates a proton to water
3. The reaction produces H₃O⁺ (or H⁺) in solution, shown by the equation NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺
4. Cl⁻ is the conjugate base of the strong acid HCl and does not accept protons from water, so it does not affect the pH; the net effect is an increase in [H₃O⁺] making pH < 7

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