Percentage yield — AQA GCSE Chemistry
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Percentage yield explained
Yield is the actual quantity of a named product formed in a reaction, usually measured in grams.
Read the full explanation
The theoretical maximum is the maximum mass of product that could possibly be created from the given reactants. Percentage yield compares the two: percentage yield = (actual yield ÷ theoretical yield) × 100. For example, if a reaction should theoretically make 8.0 g of copper sulfate but only 6.0 g is obtained, the percentage yield is (6.0 ÷ 8.0) × 100 = 75%. Losses arise from incomplete reaction, side reactions, transferring and filtering, evaporation and purification. Percentage yield is always less than 100% in practice, though a calculated value above 100% signals impure or wet product.
Your focus
- Define yield and percentage yield and identify the relevant quantities in a practical context.
- Calculate percentage yield using the actual yield and the maximum theoretical yield.
- Evaluate a percentage yield value and explain the reasons it may be below or above 100%.
Percentage yield exam tips
Marking Points
- State that yield is the actual amount of product obtained, and identify the product and unit in a given context.
- Calculate percentage yield when given the actual yield and the maximum theoretical yield.
- Apply percentage yield = (actual yield ÷ theoretical yield) × 100, rearranging to find an unknown actual or theoretical yield.
- Interpret a percentage yield value, explaining that values below 100% arise from incomplete reaction, side reactions or product loss during separation and purification.
- Recognise that a value above 100% indicates impurities or residual solvent rather than a genuine extra amount of product.
Examiner Tips
- 💡Underline the actual yield and the theoretical yield in the question before substituting, so the fraction is set up the right way round.
- 💡Show all working when substituting values into the percentage yield equation, as method marks are often available even if the final arithmetic slips.
- 💡Check whether the question asks for a mass, a percentage or a reason for a low yield, and answer in that form with units where appropriate.
Common Mistakes
- Dividing theoretical yield by actual yield instead of actual by theoretical; correct by writing the fraction as actual yield over theoretical yield before multiplying by 100.
- Forgetting to multiply by 100 and reporting a decimal such as 0.75; correct by checking that a percentage yield is expressed on a 0 to 100 scale.
- Confusing the mass of the starting reactant with the theoretical yield of the product; correct this by carefully reading the question to identify the stated theoretical maximum mass of the product.