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 yield of product is limited by the amount of the limiting reactant present. The yield of product is also affected by other factors such as experimental technique and competing side reactions.
The percentage yield compares the mass(m) or amount(n) of substance actually obtained with the mass or amount of substance that would have been predicted based on the limiting reactant (the theoretical yield).
To determine the theoretical (maximum) amount in moles of product proceed as shown below.
Calculate the amount in moles of reaction predicted for each reactant.
Divide the amount in moles of reactant used by its coefficient in the balanced equation.
Identify the maximum amount of reaction
The maximum amount of reaction is equal to the smaller amount of reaction predicted above.
The reactant that predicts the smaller amount of reaction is known as the limiting reactant.
Calculate the theoretical maximum amount in moles of product.
The yield of product is limited by the amount of the limiting reactant present. The yield of product is also affected by other factors such as experimental technique and competing side reactions.
The percentage yield compares the mass(m) or amount(n) of substance actually obtained with the mass or amount of substance that would have been predicted based on the limiting reactant (the theoretical yield).
| % yield = | n(product) actual | × 100 |
| n(product) theoretical |
| % yield = | m(product) actual | × 100 |
| m(product) theoretical |
To determine the theoretical (maximum) amount in moles of product proceed as shown below.
Calculate the amount in moles of reaction predicted for each reactant.
Divide the amount in moles of reactant used by its coefficient in the balanced equation.
| 2CH3NH2 | + | CH3COCl |  | CH3CONHCH3 | + | CH3NH3Cl | ||
| reactant used | 4 mol | 1.5 mol | ||||||
| amount of reaction predicted | 2 mol | 1.5 mol | ||||||
Identify the maximum amount of reaction
The maximum amount of reaction is equal to the smaller amount of reaction predicted above.
maximum amount of reaction = 1.5 mol
The reactant that predicts the smaller amount of reaction is known as the limiting reactant.
CH3COCl is the limiting reactant above.
Calculate the theoretical maximum amount in moles of product.
The theoretical (maximum) amount of product is the maximum amount of reaction multiplied by its coefficient in the balanced equation.
maximum amount of CH3CONHCH3 = 1.5 mol = theoretical yield
maximum amount of CH3CONHCH3 = 1.5 mol = theoretical yield