7. pH of a Solution of a Strong Base

When a strong base like NaOH is dissolved in water, nearly all NaOH-units will accept a proton:

NaOH(aq) + H₂O(l) Na⁺.H₂O(aq) + OH^-(aq)

So [OH^-] will be equal to the original concentration of the base NaOH:

For instance: 0.1 mol/L NaOH(aq) [OH^-] = 0.1 mol/L pOH = -log 0.1 = 1 pH = 13.

In this case the amount of hydroxide ions contributed by water is negligible compared with the 0.1 mol/L from the dissociation of NaOH. In pure water this amount is only 10^-7 mol/L. Due to the presence of an abundance of hydroxide ions delivered by the strong base NaOH, the water equilibrium will shift completely to the left.

Summary

pH of a solution of a strong base B in water

[OH-] = coB Calculate pOH Calculate pH