Remember that concentrations of pure solids, pure liquids, and solvents are not included in an equilibrium constant expression. If you would like a more detailed explanation, read below.

Suppose for a moment that water was included in the equilibrium expression:

Experimental results show that the position of this equilibrium does not depend on the amount of water present. How can this be explained?

The true equilibrium constant expression does not simply involve the equilibrium concentrations of the products and reactants. It involves the ratio of the equilibrium concentration for a given substance to a reference concentration for that substance (this ratio is described as the activity, or a, of the substance).

In the case of pure liquids, this reference state is the concentration of the pure liquid. Hence, the activity of water would be one and would drop out of the equilibrium constant expression:

Unless very accurate calculations are desired, it is sufficient to use equilibrium concentrations in place of activities.