3.1.3Mass changes when a reactant or product is a gas
AQA GCSE Chemistry (8462), Higher tier · Quantitative chemistry › Conservation of mass and chemical measurements
Practise Mass changes when a reactant or product is a gas. 11 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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Complete the sentence. A system that no substances can enter or leave is called a .................... system.
closed
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
(a) Some iron wool is heated strongly in an open dish on a balance. The iron reacts with oxygen from the air to form iron oxide. What happens to the reading on the balance? Tick (✓) one box.[1]
It increases, because oxygen from the air combines with the iron.
It decreases, because the iron is used up.
It stays the same, because iron oxide is a solid.
It stays the same, because no atoms are made or destroyed.
(b) Complete the sentence. A system that no substances can enter or leave is called a .................... system.[1]
(c) A reaction that produces a gas is carried out in a sealed flask on a balance. Predict what happens to the reading on the balance during the reaction. Explain your answer.[2]
Show the answer and mark scheme
(a)Answer: It increases, because oxygen from the air combines with the iron.
(b)Answer: closed
closed
(c)
the reading stays the same / does not change
the gas cannot escape from the sealed flask, so no mass leaves the system (mass is conserved)
Question 2Medium5 marks
Copper carbonate is a green powder. When it is heated it decomposes to produce black copper oxide and carbon dioxide: CuCO3(s) → CuO(s) + CO2(g) A student heated 12.35 g of copper carbonate in an open test tube until there was no further change. The mass of solid left was 7.95 g.
(a) Calculate the mass of carbon dioxide produced.[1]
(b) Explain why the mass of solid decreased. Use the equation in your answer.[2]
(c) The student repeated the experiment in a sealed container. Predict how the total mass of the container and its contents after heating would compare with the total mass before heating. Give a reason for your answer.[2]
Show the answer and mark scheme
(a)Answer: 4.40 g
4.40 (g)
(b)
carbon dioxide is produced, which is a gas / (g)
the carbon dioxide escapes into the air (so its mass is not included)
(c)
the total mass would stay the same
the carbon dioxide cannot escape / no atoms are lost or made
Question 3Hard6 marks
A teacher ignited some steel wool (mainly iron) inside a sealed flask of air on a balance. The iron reacts with oxygen: 3Fe(s) + 2O2(g) → Fe3O4(s) The flask contained only a small volume of air, so only some of the iron reacted.
(a) The reading on the balance did not change during the reaction, even though the iron gained mass. Explain why.[2]
(b) The experiment was repeated with the flask open to the air. Explain why the reading on the balance increased.[2]
(c) Predict what would happen to the mass of iron oxide formed if the experiment were repeated in a sealed flask of pure oxygen instead of air, using the same mass of steel wool. Give a reason for your answer.[2]
Show the answer and mark scheme
(a)
the flask was sealed, so no substances could enter or leave
the oxygen that combined with the iron was already inside the flask, so the total mass of the flask and its contents stayed the same
(b)
oxygen from the air outside could enter the open flask and react with the iron
the mass of this extra oxygen is added to the solid in the flask, so the total mass on the balance increases
(c)
a greater mass of iron oxide would form
there is more oxygen in the flask, so more (or all) of the iron can react before the oxygen runs out