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:

• the balanced chemical equation
• the mass and relative formula mass of the limiting reactant, and
• the relative formula mass of the product

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:
   • M_r of CaCO₃ = 40.1 + 12.0 + (3 × 16.0) = 100.1
   • M_r 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