5.11C - Calculating percentage yield
Percentage yield compares the product actually obtained with the calculated maximum. You will choose the correct masses, make their units consistent and calculate or check a percentage yield.
Actual and theoretical yield
A yield is the amount of product formed in a reaction. Percentage yield needs two product amounts: the amount actually obtained and the calculated amount that could be obtained.
Actual yield
The actual yield is the mass or amount of product obtained from the reaction or experiment.
The theoretical value is different because it comes from calculation, not from collecting and measuring the product.
Theoretical yield
The theoretical yield is the calculated maximum mass or amount of product that could be formed from the given reactants.
Both values must refer to the same product. If a question is about copper sulfate crystals, the actual yield and theoretical yield must both be for copper sulfate crystals, not for a reactant used to make them.
Most GCSE questions give both values directly. A previous part of a longer question may calculate the theoretical yield from a balanced equation, but this lesson focuses on using the actual yield and theoretical yield once they are known.
The percentage yield formula
Percentage yield is a comparison:
what was obtained / what could be obtained
Then multiply by 100 to convert the fraction into a percentage.
Percentage yield
The answer has the unit %. The actual yield and theoretical yield must use the same unit before you divide. For example, dividing g by g leaves no mass unit; multiplying by 100 gives a percentage.
A clear layout helps you check your method and can earn partial credit if you make an arithmetic slip. The number of written lines is not the number of marks. For a direct percentage-yield calculation, a correct final answer can earn full marks without showing each line; follow any explicit instruction to explain or show a particular result. You can organise your working as follows:
- Write the formula.
- Substitute the actual yield and theoretical yield.
- Do the division and multiply by
100. - Give the answer with
%, and round at the end to the requested precision, or to a sensible precision for the data.
Worked example: a reaction has a theoretical yield of 4.80 g. The actual yield is 3.84 g. Calculate the percentage yield.
Formula:
percentage yield = actual yield / theoretical yield x 100
Substitution:
percentage yield = 3.84 / 4.80 x 100
Working:
3.84 / 4.80 = 0.800
0.800 x 100 = 80.0
Answer:
percentage yield = 80.0%
Choosing the correct values
The numerator is the actual yield: the product mass obtained. The denominator is the theoretical yield: the product mass calculated or given as the maximum possible amount.
| Information in the question | Role in the formula |
|---|---|
| "mass obtained", "mass collected", "actual yield" | actual yield |
| "calculated mass", "maximum mass", "theoretical yield" | theoretical yield |
| mass of reactant used | not used directly in this formula |
| mass of product lost or left behind | not used unless the question asks for actual yield first |
For example, if a calculation predicts 10.0 g of zinc sulfate and 7.35 g is obtained, use .
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