AQA GCSE Chemistry (8462), Higher tier · Quantitative chemistry › Yield and atom economy
Practise Percentage yield. 15 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
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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
Question 3Hard6 marks
The fertiliser ammonium nitrate is made by reacting ammonia with nitric acid: NH3 + HNO3 → NH4NO3 Relative atomic masses (Ar): H = 1, N = 14, O = 16
(a) Calculate the maximum theoretical mass of ammonium nitrate that can be made from 34 tonnes of ammonia.[2]
(b) The percentage yield of the process is 95%. Calculate the actual mass of ammonium nitrate made from 34 tonnes of ammonia.[1]
(c) A factory needs to make 200 tonnes of ammonium nitrate. The percentage yield is 95%. Calculate the mass of ammonia needed. Give your answer to 3 significant figures.[3]
Show the answer and mark scheme
(a)Answer: 160 tonnes
17 tonnes of NH3 produce 80 tonnes of NH4NO3 / (34 ÷ 17) × 80
160 (tonnes)
(b)Answer: 152 tonnes
152 (tonnes)
(c)Answer: 44.7 tonnes
theoretical mass needed = 200 ÷ 0.95 = 210.5 (tonnes)