At a given temperature, a certain equilibrium reaction will always have the same ratio of products and reactants, and return to that ratio should the concentrations change. This happens by either more of the reactants reacting into products, or the products breaking down into reactants. This ratio is normally called Kc, and can be found by taking the concentration of each of the reactants or products and raising it to the power of the number of moles of that chemical. Then, set Kc equal to the concentration of the products raised to the power of moles, and multiplied by each other, divided by the reactants raised to the power of moles and multiplied by each other. Like so:
Given the Kc, this equation can also be used to find some of the concentrations.