What are buffers made of?

• A. A strong acid and conjugate base
• B. A weak base and conjugate base
• C. Made from a weak acid and conjugate base or weak base and conjugate acid
• D. A weak acid and a strong base

Answer: (C)

Buffers resist changes in pH because they contain a weak acid and its conjugate base, or a weak base and its conjugate acid. This pairing creates an equilibrium that can respond to both added acid and added base. If you add acid, the conjugate base part neutralizes the extra H+ to form the weak acid, keeping the pH from dropping much. If you add base, the weak acid donates a proton to neutralize OH−, forming the conjugate base and raising the pH only slightly. Strong acids or bases don’t form effective buffers because they dissociate completely and overwhelm the system, whereas a weak acid/base pair can hover the pH around its pKa (for acid–base pairs) or pKb, providing stable buffering action. So buffers are made from a weak acid and its conjugate base, or a weak base and its conjugate acid.