The main objective of preparative experiments is to obtain a good yield of the desired substance. It is common to use one of the reactants, usually the less expensive one, in excess to that required by the balanced equation for the reaction.
The
percentage yield compares the mass(
m) or amount(
n) of substance actually obtained with the mass or amount of substance that is the theoretical yield.
| % yield = | n(product) actual | × 100% |
| n(product) in theory |
| % yield = | m(product) actual | × 100% |
| m(product) in theory |
The amount of reaction that occurs, and therefore the amount of product formed, is limited by the reactant present in smaller amount, and this reactant is known as the
limiting reactant. As shown below, the theoretical amount of product that can be formed from a known amount of limiting reactant depends on the coefficients in the balanced equation for the reaction.
In the reaction below CH
3CO
2H is the limiting reactant,
Note that the limiting reagent determines the
change due to reaction.The
change due to reaction determines the theoretical yield of CH
3CO
2CH
3 which in this case is 1 mol.
| | CH3OH | + | CH3CO2H |  | CH3CO2CH3 | + | H2O |
| Initial amounts/mol | 2 | | 1 | | 0 | | 0 |
| Change due to reaction/mol | -1 | | -1 | | +1 | | +1 |
| Final amounts/mol | 1 | | 0 | | 1 | | 1 |