Advanced Placement (AP) Chemistry Practice Exam

What is the primary component of a buffer system?

1. A strong acid

2. A weak acid and its conjugate base

3. A neutral salt

4. A strong base

The correct answer is: A weak acid and its conjugate base

The primary component of a buffer system is indeed a weak acid and its conjugate base. A buffer functions by maintaining the pH of a solution relatively constant when small amounts of acids or bases are added. The weak acid can donate protons (H⁺) when the pH decreases (addition of an acid), while the conjugate base can accept protons when the pH increases (addition of a base). This balance allows the buffer to resist significant changes in pH, making it essential in many biological and chemical processes where stable conditions are crucial. The presence of a strong acid or strong base would lead to a more drastic change in pH rather than moderation, as strong acids completely dissociate in solution and do not provide the equilibrium necessary for buffering. A neutral salt does not have the capability to either donate or accept protons effectively, thus lacking the buffering action seen with the weak acid-conjugate base pairing.