The law of mass action introduces the equilibrium constant to relate the equilibrium concentrations of the reactants and products.

For a balanced chemical equation where A and B are reactants, C and D are products, and lowercase a, b, c, and d, are their respective stoichiometric coefficients, the equilibrium constant expression is given by the molar concentrations of the products divided by the molar concentrations of the reactants, all raised to their stoichiometric coefficients.

The equilibrium constant is often expressed with the symbol K or K_c, where the subscript c indicates the equilibrium constant with respect to molar concentration. Being a ratio of molar concentrations, the equilibrium constant is unitless.

A large equilibrium constant much greater than 1 means that the numerator, that is the concentration of the products, is greater than the denominator, the concentration of the reactants. This indicates that equilibrium lies towards the products and favors the forward reaction.

Conversely, a small equilibrium constant much less than 1 means that the denominator is greater than the numerator. This signifies that the equilibrium lies towards the reactants and favors the reverse reaction.

For an equilibrium constant of magnitude equal to or close to 1, the relative concentrations of the reactants and products are nearly the same. This indicates that neither reaction, forward nor reverse, is favored.

Modifications to the chemical equation also change the equilibrium constant.

If a chemical reaction with an equilibrium constant x is reversed, the equilibrium constant expression for the reverse reaction is the reciprocal of the forward reaction. So, the new equilibrium constant would be 1 over x.

If the coefficients are multiplied by another factor, n, the equilibrium constant is raised to the same factor. The new equilibrium constant would now be xⁿ.

When two or more individual reactions are added together, the equilibrium constant for the overall reaction is the product of the individual equilibrium constants. So, the new equilibrium constant would be y × z.

For a heterogeneous equilibrium, the equilibrium constant expression excludes pure solid or pure liquid entities.

For example, the equilibrium constant expression for the decomposition of solid ammonium nitrate to gaseous nitrous oxide and water vapor, excludes ammonium nitrate—a pure solid.

This is because the relative molar concentration of a pure solid or liquid remains constant during the reaction.