Question

A 0.150 M solution of a weak base has a pH of 11.28. Determine the Kb for the base. Express your answer using two significant figures.

Answer 1

pH + pOH = 14

pOH = 14-11.28

pOH = 2.72

[OH-] = 10^-2.72

[OH-] = 1.90 x 10^-3 M

[OH-] = x = 1.90 x 10^-3 M

B + H2O ----------------> BH+    + OH-

0.150                              0            0

0.150-x                            x           x

Kb = [BH+] [OH-] / [B]

Kb = x^2 / 0.150 -x

Kb = (1.90 x 10^-3 )^2 / 0.150 -1.90 x 10^-3

Kb = 2.44 x 10^-5