The K_a of acetic acid (HC₂H₃O₂)is 1.8 × 10^-⁵. What is the pH at 25.0°C of an aqueous solution that is 0.100 M in acetic acid?

Question

Quizplus · Accepted Answer

The pH can be calculated using the Henderson-Hasselbalch equation:
pH = pKa + log([A-]/[HA])
where pKa is the dissociation constant of acetic acid (4.76), [A-] is the concentration of acetate ion and [HA] is the concentration of undissociated acetic acid. 
At equilibrium, from the dissociation reaction:
CH3COOH ⇌ CH3COO- + H+
the concentration of [A-] and [H+] is equal, so [A-] = [H+]. The concentration of [HA] is equal to the initial concentration of acetic acid minus the concentration of [A-].
Therefore:
[A-]/[HA] = [H+]/(0.100 - [H+])
pH = 4.76 + log([H+]/(0.100 - [H+]))
Using the given value of pKa, the pH can be calculated to be 2.87. Therefore, the correct answer is A, +2.87.

The Ka of Acetic Acid (HC2H3O2)is 1

The K_a of Acetic Acid (HC₂H₃O₂)is 1