does anyone know the ka expression for the reaction acetic acid with water?

Answer 1

Taking acetic acid as HC2H3O2 with the first H being the acidic one, the reaction would be:

HC2H3O2 + H2O <=> H3O+ + C2H3O2-

and

Ka = [H3O+][C2H3O2-]
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[HC2H3O2]

The concentration of water doesn't appear because it doesn't change. Pure solids and liquid concentrations do not appear in rate or equilibrium expressions because they do not change.