Question

When salt of a weak acid is added to a solution of a weak acid at equilibirum, what would happen to the hydronium ion concentration and the hydroxide ion concentration?

Answer 1

Let us suppose salt of weak acid is CH3COONa, and solution of weak acid is CH3COOH.

This is the equilibrium reaction of the weak acid.

CH3COOH <-----------> CH3COO- + H+

When this salt is added, that means, the salt will dissolve and dissociate completely into sodium and acetate ions. From Le Chatelier's principle, we would expect these added acetate ions to force the weak acetic acid equilibrium system in the direction of less dissociation. That is the equilibrium moves to the left.

So, there will be decrease in the concentration of hydronium ions. This indirectly have effect in increasing the concentration of hydroxide ions.