Advanced Placement (AP) Chemistry Practice Exam

What is the general form of the Henderson-Hasselbalch equation?

1. pH = pKa + log([HA]/[A-])

2. pH = pKa + log([A-]/[HA])

3. pH = pKa - log([A-]/[HA])

4. pH = pKa - log([HA]/[A-])

The correct answer is: pH = pKa + log([A-]/[HA])

The Henderson-Hasselbalch equation is a crucial formula used in acid-base chemistry, especially when dealing with buffer solutions. The correct general form of the equation describes the relationship between pH, pKa, and the concentrations of the acid (HA) and its conjugate base (A-). The equation states that pH = pKa + log([A-]/[HA]). This reflects that the pH of a solution can be derived from the pKa of the weak acid involved and the ratio of the concentrations of its conjugate base to the acid itself. This form is particularly useful because it allows chemists to quickly assess how changes in the concentrations of the acid and conjugate base will affect the pH of the solution. When the concentration of the conjugate base increases relative to the acid, the log term becomes positive, which results in a higher pH, meaning a more basic solution. Conversely, if the concentration of the acid is higher relative to the conjugate base, the log term contributes negatively to the pH, indicating a more acidic environment. Using this equation allows for efficient calculations and adjustments in chemical settings, such as laboratory procedures or biochemical applications, where maintaining a specific pH is critical. Thus, the