For a given mass of reactants, it is possible to calculate the maximum mass that could be made of a certain product. This mass is called the theoretical yield. The actual yield is the mass of product made when the reaction is actually carried out.
The percentage yield is calculated using this formula:
For example, if the theoretical yield is 20 g but the actual yield is 15 g:
Percentage yield = (15 ÷ 20) × 100 = 75%
A 100 per cent yield means that no product has been lost, while a 0 per cent yield means that no product has been made. There are several reasons why the percentage yield of a product might be less than 100 per cent. These include:
Percentage purity of a substance can be calculated by dividing the mass of the pure chemical by the total mass of the sample, and then multiplying this number by 100.