The main objective of a preparative experiment is to obtain a good yield of the desired substance.
As shown below, percentage yield compares the mass(m) or amount(n) of substance actually obtained with the mass or amount of substance that would be obtained from the maximum amount of reaction.
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The maximum amount of reaction is determined by the amount in moles of limiting reactant and and its coefficient in the balanced equation for the reaction.
As shown below, percentage yield compares the mass(m) or amount(n) of substance actually obtained with the mass or amount of substance that would be obtained from the maximum amount of reaction.
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| % yield = | n(product) actual | × 100% | % yield = | m(product) actual | × 100% | |
| n(product) maximum | m(product) maximum |
The maximum amount of reaction is determined by the amount in moles of limiting reactant and and its coefficient in the balanced equation for the reaction.
In the example below the percentage yield of CH3CO2CH3 is calculated for a experiment in which the amounts shown of reactants have been added and 50 g of the product is formed.
| CH3OH | + | CH3CO2H |  | CH3CO2CH3 | + | H2O | |
| amounts added | 2 mol | 1 mol | |||||
| maximum amount of reaction | 1 mol | ||||||
| maximum amount of product formed | 1 mol | ||||||
| maximum mass of product formed | 74 g | ||||||
| 50 g of product is actually formed | percentage yield | 68% | |||||