The ion-product constant for water describes the relationship between molecular water and its dissociated ionic components at dynamic equilibrium.
Pure water dissociates partially to give the hydroxonium and hydroxide ions.
Since water is the solvent, its activity approximately equals to one and the equilibrium constant is
Kw is called the ion-product constant for water. Conductivity measurements of pure water reveals that [H3O+] = 10-7 M at 25oC. Since the concentration of hydroxonium ions and hydroxide ions are formed only from the dissociated of water, [H3O+] = [OH–], giving Kw = 10-14 M at 25oC . Note that Kw also applies to solutions of acids and bases dissolved in water, that is, Kw is still equal to 10-14 M at 25oC regardless of the source of H3O+ and OH–.
Question
What is the concentration of OH– when HCl is added to water at 25oC to give a pH of 2?
Answer
According to Le Chatelier’s principle, the increased [H3O+] shifts the position of the equilibrium of eq11 to the left to attain a new equilibrium where