Calculation of the pH of a weak base:

1. Find the [OH-] from the pKb value (in the same way as for weak acid type calculations)

2. From the [OH-] find the pOH

3. Find the pH from: pOH + pH = 14

Example:

stage 1:

pH of 0,5 M phenol (pKa = 9,83)

pKa = 9,83 therefore pKb = 14 - 9,83

pKb = 4,07

Kb = 10-4,07 = 8,51 x 10-5

Kb x [phenol] = [X+][OH-]

[OH-] = √(Kb x [phenol])

[OH-] = √(8,51 x 10-5 x 0,5)

[OH-] = 6,52 x 10-3

Stage 2:

pOH = -log10 [OH-]

pOH = 2,19

Stage 3:

pH = 14 - 2,19

pH = 11,81