Most of the time when a reaction is carried out the reactants will not be present in the proportions indicated by the balanced equation.  Consequently some reactant will be consumed while others will be left over when the reaction is finished.  The reaction used up first is called the limiting reactant.  The amount of limiting reactant present determines the amount of product which can be produced.  This amount of product produced is called the theoretical yield, it is the amount of product that would form if all of the limiting reactant is consumed in the reaction.  The theoretical yield is the maximum amount of a product that can be produced from a given amount of limiting reactant.  The amount of product actually produced by the reaction is the actual yield and will usually be less than the theoretical yield.  To express the efficiency of a reaction, chemists often calculate the percent yield, which is defined by:

Lets work through an example problem that will illustrate all of these concepts.

Example problem

Calculate the theoretical yield of C₂H₅Cl if 112 g of C₂H₅OH is reacted with 34.7 g of PCl₃ based on the reaction below.  If 23.7 g of C₂H₅Cl is produced, what is the percent yield?

Solution

First we will assume that C₂H₅OH is the limiting reactant and calculate how much C₂H₅Cl would be formed.  Then we will assume that  PCl₃ is the limiting reactant and calculate how much C₂H₅Cl would be formed.  The reactant producing the smaller amount of C₂H₅Cl is the limiting reactant and the corresponding quantity of C₂H₅Cl  is the theoretical yield.

Since the calculation with PCl₃ gives the smaller amount of C₂H₅Cl,  PCl₃ is the limiting reactant and 48.9 g is the theoretical yield of   C₂H₅Cl.  The percent yield is