Butyric acid is responsible for the odor in rancid butter. A solution of 0.25 M butyric acid has a pH of 2.71. What is the K_a for the acid?

Question

Quizplus · Accepted Answer

The dissociation constant (Ka) for butyric acid can be calculated using the formula Ka = [H+][A-]/[HA], where [H+] is the concentration of hydrogen ions, [A-] is the concentration of the acid's conjugate base, and [HA] is the concentration of the undissociated acid. Given the pH of 2.71, the [H+] can be found using the formula [H+] = 10^-pH = 10^-2.71. Assuming that the concentration of [H+] is equal to the concentration of [A-] and that the initial concentration of the acid ([HA]) is only slightly decreased by the dissociation, the Ka can be approximated as [H+]^2 / [HA]. Substituting the given values, we find that the Ka is in the order of 10^-5, matching choice D.

Butyric Acid Is Responsible for the Odor in Rancid Butter