9. pH of a Solution of a Weak Base

When a weak acid like NH₃ (ammonia) is dissolved in water, only a few NH₃-molecules will accept a proton:

NH₃(aq) + H₂O(l) NH₄⁺(aq) + OH^-(aq)

[OH^-] , pOH and pH of the solution can be calculated from K_b.

Let us calculate the pH of 0.1 mol/L NH₃.

	NH3	NH4+	OH-
Beginning	0.1 mol/L	0 mol/L (*)	0 mol/L
D	- x mol/L	+ x mol/L	+ x mol/L
Equilibrium	(0.1 - x) mol/L	x mol/L	x mol/L

(*) The hydroxide ions delivered by the ionization of water are neglected. The weak base NH₃ (K_b = 1.8 x 10^-5) is much stronger than the weak base H₂O (K_b = 1.0 x 10^-14).

So we find at equilibrium: [OH^-] = 1.3 x 10^-3 mol/L pOH = - log 1.3 x 10^-3 = 2.88 pOH = 11.12 .

Summary

pH of a solution of a weak base B in water

Calculate the concentrations in the equilibrium state Calculate pOH Calculate pH