The mass action expression is the same as the equilibrium constant expression of the reaction. The equilibrium constant expression of the reaction is the ratio of the concentration of the products and the reactants where the concentrations are being raised to the power of their coefficients from the balanced chemical reaction. Therefore, the expression for the decomposition of H2O would be:
K = </span><span>[H2]^2 [O2] / [H2O]^2
In writing equilibrium expressions, it should be noted that substances that are pure fluids are not to be included in the expression as well as solid substances.</span>
