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 25 °C a 0.100 M solution of acetic acid has the following concentrations: [HC2H3O2] = 0.0990 M, [C2H3O2 -] = 1.33 × 10-3 M, and [H+] = 1.33 × 10-3 M. The equilibrium constant, Keq, for the ionization of acetic acid at 25°C is __________.

1.75 × 10-7
1.79 × 10-5
5.71 × 106
5.71 × 104
0.100