0.1 mol of methyl amine (Kb = 5 × 10−4) is mixed with 0.08 mol of HCl and the solution is diluted to 1 litre. The hydrogen ion concentration of the resulting solution is:
[H+] = 8 × 10−11 M
CH3NH2 + HCl → CH3N+H3Cl−
At start
0.1 mol 0.08 mol 0 mol
On completion
0.02 mol 0 mol 0.08 mol
pOH = pKb + log[salt][Base] = −log(5 × 10−4) + log.08.02 = 3.9
So pH = 14 - 3.9 = 10.1
[H+] = 8 × 10−11 M