Acetic acid is a weak acid that dissociates into the acetate ion and a proton in aqueous solution: HC2H3O2 (aq) C2H3O2-(aq) + H+ (aq)

At equilibrium at a 0.100 M solution of acetic acid has the following concentrations: [HC2H3O2] = 0.0990 M, [C2H3O2-] = 1.33 x 10^-3 M, and [H+] = 1.33 x 10^-3 M. The equilibrium constant, K, for the ionization of acetic acid at is .