Acetic acid is a weak acid that dissociates into the acetate ion and a proton in aqueous solution: HC2H3O2(aq) + H2O(l) ⇌ C2H3O2-(aq) + H3O+(aq). At equilibrium at 25 °C, a 0.100 M solution of acetic acid has the following concentrations: [HC2H3O2] = 0.0990 and [C2H3O2-] = 1.33×10-3 M, and [H3O+] = 1.33×10-3 M. The equilibrium constant, Keq, for the ionization of acetic acid at 25 °C is __________.