Acid and bases question?

Water dissociates like this : H2O reversible arrow H+ + OH- . The equilibrium constant for this is Kc = [H+][OH-]/[H2O]. Because water is only very weakly dissociated [H2O] >> [H+] or [OH-] and can be taken as constant in the expression for Kc. (At 25C [H+] = [OH-] = 10^-7M and [H2O] = 1000/18 = 55.6M - if you double [H+] or [OH-], eg by changing the temperature, [H2O] = 55.6 - 2x10^-7 so hardly changed at all). So [H2O] can be incorporated into the constant part of the expression, giving Kc x [H2O] = [H+] x [OH-]. At 25C The value of Kc x [H2O] is about 10^-14, this is known as Kw