Organic acids and bases

Question

A weak acid, HA, has a p𝐾a of 4.655 . If a solution of this acid has a pH of 4.935 , what percentage of the acid is not ionized? Assume all H+ in the solution came from the ionization of HA.

Answer 1

HA <==> H⁺ + A^-

pH = pKa + log [A-]/[HA]

4.935 = 4.655 + log [A-]/[HA]

log [A-]/[HA] = 0.28

[A-]/[HA] = 1.91 This represents the ratio of A- or H+ to HA at equilibrium

[A-] / 1-[A-] = 1.91

[A-] = 1.91 - 1.91[A-]

2.91[A-] - 1.91

[A-] = 0.656 = 65.6% ionized

% not ionized = 34.4%