A buffer solution typically contains a weak acid and which of the following?

• A. A conjugate base of the weak acid
• B. A strong acid
• C. A strong base
• D. Water

Answer: (A)

A buffer works because it uses a weak acid together with its conjugate base. This pair can respond to additions of both acid and base without a large change in pH. When a little base is added, the conjugate acid donates protons to neutralize it; when a little acid is added, the conjugate base helps neutralize the extra H+ by shifting the equilibrium to form more weak acid. This reversible interplay is captured by the Henderson–Hasselbalch relation, which shows how the ratio of the conjugate base form to the weak acid form determines pH. Therefore, a typical buffer contains a weak acid and its conjugate base. A strong acid or strong base would overwhelm the system and water alone lacks the necessary conjugate pair to provide buffering capacity, so they do not make a proper buffer.