What is the pH?

Question

What is the approximate pH at the equivalence point of a weak acid-strong base titration if 75.00 mL of aqueous hydrofluoric acid requires 90.00 mL of 0.200 M KOH to reach the equivalence point? K_a = 6.76 × 10^-4 for HF.

Answer 1

The equivalence point in a titration of a weak acid with a strong base will be greater than pH 7.

HF + KOH ==> H2O + KF

HF is a weak acid and KOH is a strong base.

moles KOH used to reach equivalence = 90.00 ml x 1 L/1000 ml x 0.200 mol/L = 0.0180 mols KOH

mols KF formed = 0.0180 (see balanced equation)

Total volume = 75 ml + 90 ml = 165 ml = 0.165 L

[KF] = 0.0180 mols / 0.165 L = 0.109 M

When KF hydrolyzes we have ...

F^- + H2O ==> HF + OH^-

F^- is acting as a base so we use Kb

Kb = 1x10^-14/Ka = 1x10^-14 / 6.76x10^-4 = 1.5x10^-11

Kb = [HF][OH-] / [F-] = (x)(x) / 0.109

1.5x10^-11 = x² / 0.109

x² = 1.64x10^-12

x = 1.28x10^-6 = [OH-]

pOH = 5.89

pH = 14 - 5.89

pH = 8.1