Ion-product constant for water

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.

2H_2O(l)\rightleftharpoons H_3O^+(aq)+OH^-(aq)

Since water is the solvent, its activity approximately equals to one and the equilibrium constant is

K_w=\left [ H_3O^+ \right ]\left [ OH^- \right ]\; \; \; \; \; \; \; \; 11

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.



What is the concentration of OH when HCl is added to water at 25oC to give a pH of 2?


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

[OH^-]=\frac{10^{-14}}{10^{-2}}=10^{-12}\: M



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