Henderson-Hasselbalch Equation — Overview & Examples

(AP Chemistry 2016 Free-Response Question 4b, slightly modified)

$$\text{C}_6\text{H}_5\text{OH} (aq) + \text{H}_2\text{O} (l) \rightleftharpoons \text{C}_6\text{H}_5\text{O}^- (aq) + \text{H}_3\text{O}^+ (aq)$$

Phenol is a weak acid that partially dissociates in water according to the equation above.

For a certain reaction involving $\text{C}_6\text{H}_5\text{OH} (aq)$ to proceed at a significant rate, the phenol must be primarily in its deprotonated form, $\text{C}_6\text{H}_5\text{O}^- (aq).$ In order to ensure that the $\text{C}_6\text{H}_5\text{OH} (aq)$ is deprotonated, the reaction must be conducted in a buffer solution. On the number scale below, check each pH for which more than $50$ percent of the phenol molecules are in the deprotonated form $\left(\text{C}_6\text{H}_5\text{O}^- (aq)\right).$