ION PRODUCT OF WATER
a. Since undissociated water is present in great excess, its concentration is
virtually constant at 55.6 mol/L.
The gram molecular weight of water is 18.0; therefore, in 1.00 L
(approximately 1000 grams), there are:
---------------- = 55.6 MOL/L
b. This constant value for the concentration of water can be incorporated into
the dissociation constant to give a new constant, the ion product of water, or Kw.
1.8 X 10-16 = ----------
[H ] [OH ] = 1.0 X 10-14
Therefore, in pure water,
[H ] = 1.0 X 10-7 mol/L
[HO ] = 1.0 X 10-7 mol/L
Kw = [H+] [HO-] = 1.0 X 10-14
c. It is important to realize that the ion product of water, (H ] [OH ], is constant
for all aqueous solutions, even those that contain dissolved acids or bases.
(1) If a large number of H ions are added to pure water, the concentration
of OH ions must decrease in order that the product of [H ] [OH ] remains the same.
(2) Conversely, if a large number of hydroxyl ions are added, the [H ] will
have to decrease.