Since one mole of Fe is needed to react with one mole of S, and there are 0.5 mol Fe
versus 0.75 mol S, the limiting reagent is Fe. Thus, 0.5 mol Fe will react with 0.5 mol
S, leaving an excess of 0.25 mol S in the vessel. The mass of the excess reactant will
be:Note that the limiting reactant is not necessarily the one with the smallest mass. It depends also on
the molecular (or atomic) weights of all the reactants and also the stoichiometric relationship. In
the question above, for example, there is a higher mass of Fe than S, yet as we have seen, Fe is the
limiting reactant.
YIELDS
The yield of a reaction, which is the amount of product predicted or obtained when the reaction is
carried out, can be determined or predicted from the balanced equation. There are three distinct
ways of reporting yields. The theoretical yield is the amount of product that can be predicted from a
balanced equation, assuming that all of the limiting reagent has been used, that no competing side
reactions have occurred, and all of the product has been collected. The theoretical yield is seldom
obtained; therefore, chemists speak of the actual yield, which is the amount of product that is
isolated from the reaction experimentally.
The term percent yield is used to express the relationship between the actual yield and the
theoretical yield, and is given by the following equation:
Example: What is the percent yield for a reaction in which 27 g of Cu is produced by reacting
32.5 g of Zn in excess CuSO 4 solution?
Solution: The balanced equation is as follows:Zn (s) + CuSO 4 (aq) → Cu (s) + ZnSO 4 (aq)