H2O ----> H+ + OH-
Dissociation of water is a constant at a given temperature and a has value of 1.0 x 10^-14 at room temperature.
now dissociation constant (Kd) of above equation would give you the following:
Kd = [H+][OH-]/[H2O]
now concentration of water is a constant [H2O] = 55.18 if am not mistaken...
So you have another equation which would give you the following
Kw = [H+][OH-]
As concentration of water being a constant it would never change
Also from equation above you would notice [H+] = [OH-]
So i can simplify my equation as follows [H+]^2 = 1.0 x 10^-14
So, [H+] = 1.0 x 10^-7
now more importantly another definition of pH would be the negative logarithm to base 10 of the concentration of H+ ions in a solution.
From the second definition you obtain the formula pH = -log[H+]
So for water, pH = -log[1.0 x 10^-7]
pH = 7.
THEREFORE, pH OF WATER = 7