What does the equilibrium constant (K) represent?

The equilibrium constant, represented as K, quantifies the relationship between the concentrations of the products and the reactants in a chemical reaction at equilibrium. Specifically, it is defined as the ratio of the concentrations (or partial pressures, for gases) of the products raised to the power of their stoichiometric coefficients to the concentrations (or partial pressures) of the reactants, also raised to the power of their respective stoichiometric coefficients.

This ratio reflects the extent to which a reaction proceeds: a large K value indicates that the formation of products is favored, meaning that at equilibrium, the concentration of products is much larger than that of the reactants. Conversely, a small K value suggests that reactants predominately exist at equilibrium. This understanding allows chemists to predict the behavior of the system under various conditions, making K a fundamental aspect of chemical equilibrium.

Other choices do not accurately define K; the total concentration of all reactants and the total pressure of all gases do not provide the necessary information about the ratio of products to reactants. Similarly, the sum of stoichiometric coefficients does not convey any concentration or pressure relationship relevant to the equilibrium state.