A hydrolysis constant is an equilibrium constant for a hydrolysis reaction. For example, if a metal salt such as AlCl3 dissolves in an aqueous solution, the metal cation behaves as a Lewis acid and hydrolyzes the water molecules in the solvent. Al3+ + 2H2O → AlOH2+ + H3O+ The hydrolysis constant for this reaction is as shown: Khydrolysis = [H3O+] * [AlOH2+] / [Al3+] In a more generalized form, the hydrolysis constant can be described as: Ka = [H3O+] * [A−] / [HA] where A− represents any base, and HA represents any acid.