Answer to Question #278136 in General Chemistry for Tee

Question

The pH of a 0.160 M solution of a weak base is 9.21. What is the Kb of the base?

Answer 1

If pH = 9.21

pOH = 14.0 - 9.21 = 4.79

Then [OH-] = 10^-4.79

[OH-] = 6.31*10^-5

This is low enough to allow the use of the short cut method

If the base is MOH

Then Kb = [M+] [OH-] / [MOH]

[M+] = [OH-] and [MOH] = 0.1 M

Kb = (6.31*10^-5)² / 0.1

Kb = 3.98*10^-9 / 0.1

Kb = 3.98*10^-8