Theoretical Yield - Calculating Yields - OCR 21C - BBC Bitesize - BBC

In this guide

1. Revise
2. Test

Pages

1. Theoretical yield
2. Percentage yield
3. Avogadro’s law - Higher
4. The molar value of a gas - Higher
5. Molar volume calculations

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 carbonate 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

More guides on this topic

• Separating chemical mixtures
• Finding the composition of unknown samples
• Calculating amounts of substances
• Measuring the amounts of chemicals in solutions
• Sample exam questions - chemical analysis

Related links

• Chemistry: Exam practice
• Chemistry revision resources
• Bitesize revision podcasts
• Personalise your Bitesize!
• Jobs that use Chemistry
• Save My Exams Subscription
• Quizlet
• Royal Society of Chemistry
• Revisio Subscription
• Science Museum