What is the ion-product expression constant for water, Kw and why is H2O absent from this expression?
The ion-product expression for Kw is as follows
Kw = [H3O+][OH-]
where [H3O+] is the concentration of H3O+ and [OH-] is the concentration of OH-. At 25 degC, the value for Kw is 1.00 x 10^-14. Like all equilibrium constants, this will change with temperature but most measurements are taken at 25 degC.
Pure liquids and solids are omitted from equilibrium expressions as they do not have a concentration so it doesn't change when additional amounts are added. As a result, a change in the amount of either a pure liquid or the solid does not affect the value of K and is omitted from equilibrium expressions whether we are looking at the ion-product expression of water or at the equilibrium constant of a chemical reaction.