Chemistry: The Molecular Science (5th Edition)

$K=\frac{[HCl]^2}{[H_2O]}$
Recall, in the expression for the equilibrium constant, the concentrations of the reactants are in the numerator, and each is raised to an exponent that is the same as the coefficient in front of each reactant, and then these values are multiplied by each other. The same thing happens with the products, which are in the denominator. Recall, solids are not included in the expression for the equilibrium constant. Keeping this in mind, we obtain: $K=\frac{[HCl]^2}{[H_2O]}$