AQA GCSE Chemistry Foundation (8462), Foundation tier · Quantitative chemistry › Yield and atom economy
Practise Percentage yield. 6 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
The yield is the amount of product you actually obtain. Percentage yield compares it with the maximum (theoretical) amount that could be made. This is in GCSE Chemistry only, not Combined Science.
Key facts
Yield = amount of product actually obtained
% yield = actual mass ÷ theoretical mass × 100
Actual mass = % yield × theoretical mass ÷ 100
Low yield: reversible reaction; product lost when separated; reactants react in unexpected ways
A percentage yield cannot be more than 100% (if it is, the product is probably impure or still wet)
Notes
Yield and percentage yield
diagram
The yield is the amount of product obtained.
The theoretical yield is the maximum mass of product that could be made, calculated from the balanced equation.
percentage yield = (mass of product actually made ÷ maximum theoretical mass of product) × 100
Percentage yield = 10.0 ÷ 12.5 × 100 = 80.0%. The missing 2.5 g is the product that was not obtained.
Why the yield is less than 100%
No atoms are gained or lost, but it is not always possible to obtain the calculated amount of product because:
the reaction may not go to completion because it is reversible
some of the product may be lost when it is separated from the reaction mixture (e.g. left on the filter paper or in the apparatus)
some of the reactants may react in ways different to the expected reaction.
A high percentage yield wastes fewer reactants and less energy, which saves resources and money.
How to answer each type of question
Calculate the percentage yield
2 marksGrade 5
Divide the actual mass by the theoretical (maximum) mass.
Multiply by 100 and give the % sign.
Example. The maximum theoretical mass of magnesium sulfate that could be made in a reaction was 6.00 g. A student obtained 4.20 g of dry crystals. Calculate the percentage yield. [2 marks]
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4.20 ÷ 6.00 × 100 (1) = 70.0% (1)
Give reasons why the yield is less than 100%
2 marksGrade 5
Choose from the three standard reasons.
Make each reason specific and link it to the context if one is given.
Example. Give two reasons why the percentage yield of a product is less than 100%. [2 marks]
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Any two from: the reaction is reversible, so it does not go to completion (1) some product is lost when it is separated from the reaction mixture (1) some reactants react in ways different to the expected reaction (1)
Don’t lose marks
Putting the theoretical mass on top and getting an answer over 100%.
Confusing percentage yield with atom economy: yield uses measured masses; atom economy uses Mr values from the equation.
Giving vague reasons such as 'human error' or 'mistakes' instead of the three standard reasons.
Saying that atoms are lost. Atoms are never lost; product is lost from the apparatus or never forms.
More tips
Memory tricks
Actual over theoretical: what you got goes on top, so the answer is below 100%.
Remember the reasons as R, L, S: Reversible, Lost when separating, Side (unexpected) reactions.
Quick check: a 50% yield is half the theoretical mass.
A 100% yield would mean every bit of reactant became product and none was lost.
Exam technique
Learn the three reasons for a low yield word for word; each is often worth a mark.
Write the division before multiplying by 100 so you can earn the method mark.
Give the % sign and round only at the end.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 4
State what is meant by yieldThe amount of product obtained from a reaction.
Grade 5
Calculate percentage yield% yield = mass of product actually made ÷ maximum theoretical mass × 100.
Grade 5
Give reasons why yield is below 100%The reaction is reversible, some product is lost when it is separated, or some reactants react in unexpected ways.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
A reaction has a maximum theoretical yield of 12.0 g. A student obtains 9.0 g of product. Calculate the percentage yield.
75%
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
The percentage yield of a reaction can be calculated using the equation: percentage yield = (mass of product actually made ÷ maximum theoretical mass of product) × 100
(a) A student made 4.2 g of a product. The maximum theoretical mass of the product was 5.0 g. Calculate the percentage yield.[2]
(b) Give two reasons why the percentage yield of a reaction is often less than 100%.[2]
Show the answer and mark scheme
(a)Answer: 84%
(4.2 ÷ 5.0) × 100
84 (%)
(b)
the reaction may be reversible / may not go to completion
some of the product may be lost when it is separated from the reaction mixture
some of the reactants may react in ways different from the expected reaction / side reactions
Question 2Medium5 marks
A student made crystals of zinc sulfate from zinc oxide and dilute sulfuric acid. The maximum theoretical mass of zinc sulfate crystals was 16.1 g. The student made 11.3 g of crystals.
(a) Calculate the percentage yield. Give your answer to 3 significant figures.[2]
(b) Suggest two ways in which some of the zinc sulfate could have been lost during the practical.[2]
(c) Ammonia is made by the Haber process: N2 + 3H2 ⇌ 2NH3 Explain why the yield of ammonia can never be 100%.[1]
Show the answer and mark scheme
(a)Answer: 70.2%
(11.3 ÷ 16.1) × 100
70.2 (%)
(b)
some solution left in the beaker / soaked into the filter paper during filtering
some solution spat out / splashed during heating / evaporation
some crystals left on the filter paper or stuck to the apparatus
not all of the zinc sulfate crystallised / some stayed dissolved in the remaining solution
(c)
the reaction is reversible, so it does not go to completion / some ammonia breaks down back into nitrogen and hydrogen