GCSE Chemistry Revision — Percentage Yield
Revise Percentage Yield for GCSE Chemistry with a topic explanation, worked example and common mistakes. Check the board notes for specification differences.
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What is Percentage Yield?
Percentage yield is a measure of the efficiency of a chemical reaction. It compares the actual amount of product obtained (the actual yield) with the maximum amount that could have been formed (the theoretical yield). A yield of 100% means no product was lost.
Board notes: Percentage yield is a higher-tier topic for AQA, Edexcel, and OCR. It combines practical considerations with calculation skills. You need to understand the reasons for loss of yield and be able to perform the calculation.
Step-by-step explanationWorked examples
Worked example
In a reaction, the theoretical yield of a product is 15g. A student carries out the experiment and obtains 12g of the product. The percentage yield is (12g / 15g) x 100 = 80%.
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Common mistakes
- 1Calculating the theoretical yield incorrectly. This must be calculated from the limiting reactant using stoichiometry.
- 2Mixing up the actual yield and theoretical yield in the formula. Remember: Percentage Yield = (Actual Yield / Theoretical Yield) x 100.
- 3Forgetting that yield can be reduced by incomplete reactions, side reactions, or loss of product during separation and purification.
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Frequently asked questions
Why is the percentage yield never 100%?
In practice, it's very difficult to achieve 100% yield. Some product may be lost when it is transferred between containers, the reaction may be reversible, or unexpected side reactions may occur.
Can the percentage yield be over 100%?
A percentage yield over 100% usually indicates an error. This could be due to the product being impure, for example, if it is not completely dry when weighed.