Barrons SAT Subject Test Chemistry, 13th Edition

Sample Problem

(This sample incorporates the entire discussion of dissociation constants,
including finding the pH.)

Calculate (a) the [H 3 O+], (b) the pH, and (c) the percentage dissociation for

0.10M acetic acid at 25°C. The symbol Ka is used for the acid dissociation

constant. Ka for HC 2 H 3 O 2 is 1.8 × 10−5.

(a) For this reaction

H 2 O(l) + HC 2 H 3 O 2 (l) H 3 O+(aq) + C 2 H 3 O 2 −(aq)

and

Let x = number of moles/liter of HC 2 H 3 O 2 that dissociate and reach equilibrium.

Then

[H 3 O+] = x, [C 2 H 3 O 2 −] = x, [HC 2 H 3 O 2 ] = 0.10 − x

Substituting in the expression for Ka gives

Because a weak acid, such as acetic, at concentrations of 0.01M or greater
dissociates very little, the equilibrium concentration of the acid is very nearly
equal to the original concentration, that is,

0.10 − x ≅ 0.10

Therefore, the expression can be changed to

(b) Substituting this result in the pH expression gives