The Strength of an Acid Is Specified by Its Dissociation Constant. The equilibrium constant for an acid–base reaction is expressed as a dissociation constant with the concentrations of the “reactants” in the denominator and the concentrations of the “products” in the numerator: [2-4] In dilute solutions, the water concentration is essentially constant, 55.5 M (1000 g ⴢ L ⫺1 兾18.015 g ⴢ mol ⫺1 ⫽ 55.5 M). Therefore, the term [H 2 O] is custom- arily combined with the dissociation constant, which then takes the form [2-5] For brevity, however, we will henceforth omit the subscript “a.” The dissociation constants of some common acids are listed in Table 2-4. Because acid dissociation constants, like [H ⫹ ] values, can be cumbersome to work with, they are transformed to pK values by the formula [2-6] which is analogous to Eq. 2-3. pK ⫽⫺log K K a ⫽ K 3H 2 O4⫽ 3H ⫹ 43A ⫺ 4 3HA4 K ⫽ 3H 3 O ⫹ 43A ⫺ 4