At equilibrium, the product of the activities (or concentrations) of the reacting species is constant. Thus for the equilibrium: \[\alpha\,\rm{A}\,+\,\beta\,\rm{B}\rightleftharpoons \gamma\,\rm{C}\,+\,\delta\,\rm{D}\] \[K=\frac{\left[\rm{C}\right]^{\gamma }\ \left[\rm{D}\right]^{\delta }}{\left[\rm{A}\right]^{\alpha }\ \left[\rm{B}\right]^{\beta }}\]