Percentage Yield
Master Percentage Yield for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Percentage yield = (actual mass of product ÷ theoretical mass) × 100, and is never 100% because of losses and reversible reactions.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
The percentage yield compares the mass of product actually obtained with the maximum mass that could theoretically be made.
The formula is percentage yield = (mass of product actually made ÷ maximum theoretical mass) × 100. The answer can never exceed 100 per cent, and a value above that indicates the product was impure or still wet.
Yields are less than 100 per cent for three reasons: the reaction may be reversible and not go to completion, some product may be lost when it is separated from the reaction mixture, and there may be unexpected side reactions producing different products.
Revision notes
The calculation
Percentage yield = (actual mass of product ÷ maximum theoretical mass) × 100.
The theoretical mass is calculated from the balanced equation using reacting masses. The percentage yield can never exceed 100 per cent.
Why yields are below 100 per cent
The reaction may be reversible and therefore not go to completion.
Some of the product may be lost when it is separated from the reaction mixture, for example during filtering. There may be unexpected reactions taking place producing different products.
Why high yields matter
A higher percentage yield means less raw material is wasted, so the process is cheaper and more sustainable.
Industrial processes are designed to maximise yield where possible, which links this topic to atom economy and to reversible reactions.
Key points
- Percentage yield compares actual with theoretical mass.
- Yield = (actual ÷ theoretical) × 100.
- Yield can never exceed 100 per cent.
- Reversible reactions reduce yield.
- Product may be lost during separation.
- Side reactions produce different products.
Worked examples
Example 1
A reaction has a theoretical yield of 20 g but only 15 g is obtained. Calculate the percentage yield. [2 marks]
Model answer
Percentage yield = (15 ÷ 20) × 100 (1 mark) = 75 per cent (1 mark).
Example 2
Give two reasons why the percentage yield of a reaction may be less than 100 per cent. [2 marks]
Model answer
The reaction may be reversible and so not go to completion (1 mark). Some of the product may be lost when it is separated from the reaction mixture (1 mark).
Example 3
A student calculates a percentage yield of 105 per cent. Suggest what has gone wrong. [2 marks]
Model answer
The product may still contain impurities or be wet, adding to its mass (1 mark), because the yield can never genuinely exceed 100 per cent (1 mark).
Common mistakes
Getting the fraction upside down.
It is actual ÷ theoretical, so the answer is at most 100 per cent.
Accepting a yield above 100 per cent.
This is impossible and indicates impurities or moisture.
Forgetting to multiply by 100.
The answer is a percentage, not a fraction.
Giving only one reason for a low yield.
Three reasons are examinable — reversibility, losses and side reactions.
Exam tips
- Write the formula before substituting.
- Check the answer is 100 per cent or below.
- Learn all three reasons for reduced yield.
- Link high yield to lower cost and sustainability.
Key terms
- Percentage yield
- The actual mass obtained as a percentage of the theoretical maximum.
- Theoretical yield
- The maximum mass of product calculated from the equation.
- Reversible reaction
- A reaction that can proceed in both directions.
- Side reaction
- An unexpected reaction producing a different product.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.