A 0.6 M solution of a weak acid had a pH of 5.8. What is the pK_a of the solution?

Question

Quizplus · Accepted Answer

Since the solution is a weak acid, we can use the Henderson-Hasselbalch equation: 
pH = pKa + log([A-]/[HA])
where [A-]/[HA] is the ratio of the concentrations of the conjugate base and the acid, respectively.

From the problem, we know that [HA] = 0.6 M.

To find [A-], we can use the fact that the solution is weak, which means that the amount of [H+] released is approximately equal to the amount of [HA] that dissociates. Therefore, we can assume that [H+] = [A-].

From the pH, we can find [H+]:
pH = -log[H+]
5.8 = -log[H+]
[H+] = 10^-5.8 = 1.58 x 10^-6 M

Since [H+] = [A-], [A-] = 1.58 x 10^-6 M.

Substituting these values into the Henderson-Hasselbalch equation:
5.8 = pKa + log(1.58 x 10^-6/0.6)
pKa = 10.5

Therefore, the pKa of the solution is 10.5.

A 06 M Solution of a Weak Acid Had a PH