For a general reaction, in which a moles of substance A and b moles of B react to form c moles of C and d moles of D,

a A + b B c C + d D

the equilibrium-constant expression is




This is the law of mass action.

Concentrations can be measured in any terms that express the relative numbers of molecules per unit volume. In solutions, concentrations usually are given in molarity, or moles per liter, . This is indicated by a subscript C to the equilibrium constant: . Gases sometimes are measured in moles per liter also, but more often are measured in mole fractions, , or in partial pressures, . (The subscript j simply refers to any species.) The equilibrium constants using these units are indicated by and .

The mole fraction of the jth component of a gas mixture, , is the number of moles of that particular gas, divided by the total number of moles of all gases present:




Our atmosphere has one molecule of for every four molecules of ; thus the mole fraction of oxygen gas is




and that of nitrogen is




The sum of mole fractions of all the gases in a mixture, of course, must be exactly one.