1. Self-ionization of water
The self-ionization of water (also autoionization of water, and autodissociation of water) is an ionization reaction in pure water or an aqueous solution.

2. In this process
water molecule, H2O, deprotonates(loses the nucleus of one of its hydrogen atoms) to become a hydroxide ion, OH−.
The hydrogen nucleus, H+, immediately  protonates another water molecule to form  hydronium, H3O+

3. Water molecules can function as both acids and bases
Water molecules can function as both acids and bases. One water molecule (acting as a base) can accept a hydrogen ion from a second one (acting as an acid).
 A hydronium ion and a hydroxide ion are formed.

5. However, the hydroxonium ion is a very strong acid, and the hydroxide ion is a very strong base. As fast as they are formed, they react to produce water again. The net effect is that an equilibrium is set up.
2H2O(l)⇌H3O+(aq)+OH−(aq)

6. H2O + H2O---------->  H3O+ + OH−
Approximating activities by concentrations, the chemical equilibrium constant, Keq, for this reaction is given by:
The ionization constant, dissociation constant, self-ionization constant, or ionic product of water, symbolized by Kw may be given by:
Kw=[H3 O+ ][OH − ]
where [H3O+] is the concentration of hydrogen or hydronium ion, and [OH−] is the concentration of hydroxide ion.

7. The reaction does not form very much H3O+ or OH-
The reaction does not form very much H3O+ or OH-. In one liter of water there are about 55 moles of water molecules, but only 1.0 x 10-7 moles of H3O+ and OH- are formed (at room temperature).
So the concentrations of H3O+ and OH- in pure water are 1.0 x 10-7 M. It is the 7 in the exponent or power of this number that gives neutral water a pH of 7.

8. Equilibrium constant
Pure water will dissociate to form equal concentrations 
[H3O+]=[OH−]  we can find K, the equilibrium constant.
K=[H3O+][OH−]
At standard temperature and pressure (STP), the equilibrium constant of water, Kw, is equal to 
Kw=[H3O+][OH−]
Kw=[1.0×10−7][1.0×10−7]
Kw=1.0×10−14

9. Calculation
In this equation [H3O+] is the concentration of hydronium ions, which in a chemical equation is the acid constant, Ka. The [OH-] is the concentration of hydroxide ions, which in a chemical equation is the base constant, Kb. If given a pH, then you can easily calculate the [H3O+] by simply taking the negative reverse log of the pH:
[H3O+]=10−pH.
The same formula applies to obtaining [OH-] from the pOH:
[OH−]=10−pOH
Adding the pH's gives you the pKw
pKw=pH+pOH=14.00

10. Example
If a solution has a pH of 2.1, determine the concentration of hydroxide ion, [OH-].
SOLUTION
 To solve for this, you must first determine the concentration of the hydronium ion, [H3O+]:
[H3O+]    =10-pH
=10-2.1
=7.94 x 10-3
 Then, you solve for [OH-] using the Kw constant:
Kw = [H3O+] [OH-]
1.0 x 10-14 = [OH-][7.94 x 10-3]
[OH-] = (1 x 10-14)/ (7.94 x 10-3)
= 1.26 x 10-12

11. Example
If a solution has a pOH of 11.2, determine the concentration of hydronium ion, [H3O+].
SOLUTION
To solve for this, you must first determine the concentration of the hydroxide ion, [OH-]:
[OH-]=10-pOH = =6.31 x 10-12
Then, you solve for [H3O+] using the Kw constant:
Kw = [H3O+] [OH-] 1.0 x 10-14 = [H3O+][6.31 x 10-12] [H3O+]= (1 x 10-14)/ (6.31 x 10-12)=

12. URL en.wikipedia.org/wiki http://dl.clackamas.cc.or.us/