Question

A 0.100 M solution of a weak base has a pH of 9.75. What is the base hydrolysis constant, Kb, for the weak base?

Answer 1

first find the OH and then find the concentration of OH- using pH value

pH + pOH = 14

pOH = 14 -pH = 14 - 9.75 = 4.25

[OH-] = 10^-pOH = 10^ -4.25 = 5.6 x 10^-5

lets week base = B construct ICE table

B (aq) + H2O (l) <-----> BH (aq) + OH- (aq)

I 0.1 0 0

C -x +x +x

E 0.1-x +x +x

if 1 mole of base under go hydrolysis it will give on emole of BH and 1 mole of OH-

that means concentration of OH- and concentration of BH are equal

x = 5.6 x 10^-5 = [OH-] = [BH]

[B] at equilibrium = 0.1-x = 0.1 - 5.6 x 10^-5 = 0.099944 M

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

plug in all these values

Kb = [5.6 x 10^-5] [5.6 x 10^-5] / [0.099944] = 3.14 x 10^-8