Theoretical yield

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.

Worked example

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.

  1. Write down the balanced chemical equation:
    • CaCO3 \rightarrow CaO + CO2
  2. 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
  3. 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
  4. Work out the reacting masses for 1 g (or 1 kg or 1 tonne if different mass units are used):
    • \frac{100.1}{100.1} = 1 g of CaCO3 produces \frac{56.1}{100.1} g of CaO
  5. Scale up the reacting masses to match the given reacting mass of the reactant:
    • 250 g of CaCO3 produces \frac{56.1}{100.1} x 250
    • = 140 g of CaO