The acid and base equilibrium constant expressions describe the relationship between the amounts of reactants and products in aqueous acid-base systems. For the following general weak-acid equilibrium:

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the equilibrium constant expression is:

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For a general weak-base equilibrium:

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the equilibrium constant expression is:

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The concentration of water (or any pure liquid or solvent or solid) does appear in the equilibrium constant expression. K is the equilibrium constant, the subscript b indicates that this is an equilibrium constant expression for a weak base, and the brackets indicate molar concentrations.