99.
ION PRODUCT OF WATER
a. Since undissociated water is present in great excess, its concentration is
virtually constant at 55.6 mol/L.
NOTE:
The gram molecular weight of water is 18.0; therefore, in 1.00 L
(approximately 1000 grams), there are:
(1000 g)

(18.0 g/mol)
 = 55.6 MOL/L
1L
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.
[H+] [OH]
1.8 X 1016 = 
[55.6]
+

[H ] [OH ] = 1.0 X 1014
Therefore, in pure water,
+
[H ] = 1.0 X 107 mol/L

[HO ] = 1.0 X 107 mol/L
Kw = [H+] [HO] = 1.0 X 1014
+

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.
MD0837
98