# Explain why a mixture of ammonia solution and ammonium chloride solution acts as a buffer, maintaining a nearly constant pH when a small amount of strong acid is added.

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

## Mark scheme (4 marks)

1. A buffer contains a weak acid and its conjugate base (or a weak base and its conjugate acid) in significant concentrations.
2. When a small amount of strong acid (H⁺/H₃O⁺) is added, the base component (NH₃) reacts with and removes the added protons.
3. The equilibrium NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ (or NH₄⁺ ⇌ NH₃ + H⁺) shifts to maintain the ratio of weak acid to conjugate base, so [H⁺] (and hence pH) remains nearly unchanged.
4. The system works because the concentrations of both NH₃ and NH₄⁺ are large relative to the amount of added acid, so the ratio [NH₄⁺]/[NH₃] (and therefore pH) changes only minimally.

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