When an equation is balanced, the coefficients of the compounds dictate the relative amounts of the reactants needed to complete the reaction.
For example, in the preceding equation (1), every mole of oxygen gas requires two moles of solid copper. If the reaction is started with two moles of copper and an unlimited supply of oxygen gas, the reaction will end when the copper supply is exhausted. In this case, copper is the limiting reagent. The leftover oxygen gas is known as the excess reagent.
In problems involving limiting reagents, it is often necessary to determine which of the reactants is limiting. In such problems, the masses of the reagents are generally given. These values can then be converted to moles. Since the balanced equation provides the correct mole ratio, the given mole values can be used to determine which of the reagents is limiting.
