Which pair is typically present in a buffer?

• A. A weak acid and its conjugate base
• B. A strong acid and a strong base
• C. A salt and water
• D. A strong base and a weak acid

Answer: (A)

Buffers resist pH changes by having a weak acid and its conjugate base present in solution. The weak acid can donate protons to neutralize added base, while the conjugate base can react with added acid, forming the weak acid again. This back-and-forth keeps the pH near the acid’s dissociation constant and is conveniently described by the Henderson–Hasselbalch relation. Because strong acids and bases dissociate completely, they don’t provide the same stabilizing buffer action; a salt and water is just a solution of ions without the necessary conjugate pair in equilibrium, and a strong base with a weak acid lacks the paired conjugate components needed to cushion pH changes.