Wed January 16, 2013 By:

What is the reason for the self dissociation of water?

Expert Reply
Wed January 16, 2013
Water molecules collide with one another to cause the self-ionization reaction represented by this equation: 2H2 H3O+ OH-
It is a reversible reaction so the equation is usually written with the arrows going in both directions: 2H2 H3O+ 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.
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