Why is water often ignored in the autoionization of H2O?

[lha08]

Homework Statement

i was wondering if anyone knows why water is ignored in the autoionization of water? thanks

Homework Equations

H2O+H2O= H3O+OH
Kc=[H][OH]

The Attempt at a Solution

 

[Ygggdrasil]

In an aqueous solution (i.e. a solution where water is the solvent), the concentration of water is about 55M. Any reactions that occur in water that use water as a reactant or product will not significantly change the concentration of water in the solution. Therefore, one can treat [H₂O] as a constant that gets added into the equilibrium constant.