The equilibrium constant expression for this reaction that takes place in water and involve ions can be written as K= [H_3 O^+ ][OH^- ]/([H_2 O] ^2 ). But the concentration of undissociated water, H_2 O is much larger than the concentration of the ions that is essentially remains constant. Therefore, we can include it in the equilibrium constant. The resulting new equilibrium constant can be written: K_W= [H_3 O^+ ][OH^- ].