If there are two possible reactions for the same reactants, the reacting masses can be used to identify which reaction has taken place.For example, copper forms two oxides CuO and Cu2O, copper(II) oxide and copper(I) oxide.An oxide of copper is heated in a stream of hydrogen to constant mass.This forms copper and water. By working out the ratio of moles of copper to the ratio of moles of oxygen from their masses, the formula of the copper oxide can be found.