Phenylacetic acid is one of the substances that accumulates in the blood of persons with phenylketonuria, an inherited disorder that can cause mental retardation or even death. A 0.085 M solution of this substance is found to have a pH of 2.68. What is the acid ionization constant , K _a, for this acid?

Question

Quizplus · Accepted Answer

To find the acid ionization constant (Ka), first calculate the concentration of H+ ions using the pH: [H+] = 10^(-2.68) = 2.09×10^(-3) M. For a weak acid HA dissociating as HA ⇌ H+ + A-, the initial concentration is 0.085 M, and the change in concentration is x = [H+] = 2.09×10^(-3) M. The equilibrium concentration of HA is 0.085 - x ≈ 0.085 M. Ka = [H+][A-]/[HA] = (2.09×10^(-3))^2 / 0.085 ≈ 5.3×10^(-5).

Phenylacetic Acid Is One of the Substances That Accumulates in the Blood