[kochibacha]

In this equation

CH3COOH + H2O <----> CH3COO^- + H3O⁺

if i wanna calculate the concentration of CH3COOH or CH3COO or H3O  why don't i have to use concentration of H2O?

would you give me an example which has a distinctive result of not having concentration of H2O constant and having concentration of H2O constant

[Borek]

Water concentration is assumed to be constant and is already included in the value of acid dissociation constant.

[kochibacha]

Water concentration is assumed to be constant and is already included in the value of acid dissociation constant.

Would you please explain "WHY WATER CONCENTRATION(SOLID AS WELL) IS ASSUME TO BE CONSTANT"

I'm very curious

[Borek]

Solids are a different case.

Try to calculate concentration of water in 0.01M and 0.1M solution of acetic acid.

[Vidya]

Water or any solvent concentration term is taken unity because it is present in excess.So it is taken as change in concentration of any of the solvent can not effect the equilibrium constant.
Moreover concentration is the term which gives the amount of the solute in the given amount of the solvent and is for solutions
It is not applicable on pure liquids and solids.