The theoretical yield is the maximum possible mass of a product that can be made in a chemical reaction.
It can be calculated from:
An actual yield is the mass of a product actually obtained from the reaction. It is usually less than the theoretical yield. The reasons for this include:
- incomplete reactions, in which some of the reactants do not react to form the product
- practical losses during the experiment, such as during pouring or filtering
- side reactions (unwanted reactions that compete with the desired reaction)
- reversible reactions
- impurities in reactants
Calculating theoretical yield
Reacting masses may be used to calculate the theoretical yield. Theoretical yield can also be worked out using a mole calculation.
If heated, calcium oxide decomposes to form calcium oxide and carbon dioxide.
Calculate the maximum theoretical yield of calcium oxide that can be produced from 250 g of calcium carbonate.
- Write down the balanced chemical equation:
- CaCO3 CaO + CO2
- Calculate the relative formula masses of the substances in the question:
- Mr of CaCO3 = 40.1 + 12.0 + (3 × 16.0) = 100.1
- Mr of CaO = 40.1 + 16.0 = 56.1
- Use the balanced chemical equation to work out the reacting masses based on the relative formula masses:
- 100.1 g CaCO3 produces 56.1 g of CaO
- Work out the reacting masses for 1 g (or 1 kg or 1 tonne if different mass units are used):
- = 1 g of CaCO3 produces g of CaO
- Scale up the reacting masses to match the given reacting mass of the reactant:
- 250 g of CaCO3 produces x 250
- = 140 g of CaO