AQA GCSE Chemistry
AQA GCSE Chemistry 4.3.1 Conservation of Mass and Chemical Measurements Exam Questions
Free AQA GCSE Chemistry exam questions for 4.3.1 Conservation of Mass and Chemical Measurements, with a mark scheme for every pack. Covers conservation of mass, balanced equations, relative formula mass, apparent mass changes involving gases, and uncertainty in chemical measurements.
4.3.1 Conservation of Mass and Chemical Measurements exam questions
3 question packs of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About AQA GCSE Chemistry 4.3.1 Conservation of Mass and Chemical Measurements
AQA GCSE Chemistry 4.3.1 is the starting point for quantitative chemistry. You learn that no atoms are lost or made in a chemical reaction, so mass is conserved, and you use this to balance equations and explain why some reactions appear to gain or lose mass. You also calculate relative formula mass and estimate uncertainty from a range of results. It sits in section 4.3, examined on Paper 1, and underpins every calculation in moles, yield and concentration. Examiners often show an open flask on a balance and ask you to explain a mass change, or give repeat readings and ask for a mean and its uncertainty.
What you need to know
- Conservation of mass The total mass of the products equals the total mass of the reactants because atoms are rearranged, not created or destroyed. This is why equations must be balanced, with the same number of each type of atom on both sides.
- Relative formula mass Mr is the sum of the relative atomic masses of the atoms in the formula. In a balanced equation, the sum of Mr of reactants (allowing for the numbers in front of formulae) equals the sum of Mr of products.
- Apparent mass changes If a reactant is a gas, for example oxygen when a metal is heated in air, the mass of the solid increases. If a product is a gas that escapes, for example carbon dioxide from heating a carbonate, the mass in an open container decreases.
- Uncertainty Any measurement has uncertainty. The range of repeat results about the mean gives an estimate of uncertainty: uncertainty is roughly plus or minus half the range.
- Balancing equations Only change the numbers in front of formulae, never the small numbers within a formula, and check every element including oxygen at the end.
Common mistakes in 4.3.1 Conservation of Mass and Chemical Measurements
- Saying mass is 'lost' or 'destroyed' when a carbonate is heated. The mass is conserved; carbon dioxide gas escapes into the air, so the measured mass falls.
- Changing subscripts to balance an equation, for example writing H2O2 instead of 2H2O.
- Forgetting to multiply by the number of atoms when working out Mr, for example Ca(OH)2 = 40 + 2 x (16 + 1) = 74.
- Giving uncertainty as the whole range. The uncertainty is usually plus or minus half the range.
- Including anomalous results in a mean. Identify and exclude them first.
Try three 4.3.1 Conservation of Mass and Chemical Measurements exam-style questions
1. Calculate the relative formula mass of calcium carbonate, CaCO3. (Ar: Ca = 40, C = 12, O = 16) (1 mark)
Show the mark scheme answer
40 + 12 + (3 x 16) = 100.
2. Magnesium ribbon is heated in an open crucible and its mass increases. Explain why. (2 marks)
Show the mark scheme answer
Magnesium reacts with oxygen from the air; forming magnesium oxide, so the mass of oxygen atoms is added to the solid.
3. A student measures the time for a reaction three times: 12.1 s, 12.5 s and 12.3 s. Calculate the mean and estimate its uncertainty. (2 marks)
Show the mark scheme answer
Mean = (12.1 + 12.5 + 12.3) / 3 = 12.3 s; range = 0.4 s so uncertainty = plus or minus 0.2 s.
4.3.1 Conservation of Mass and Chemical Measurements FAQs
What does AQA GCSE Chemistry 4.3.1 cover?
It covers the law of conservation of mass, balancing equations, relative formula mass, why reactions involving gases can seem to change mass, and estimating uncertainty in measurements.
Which paper is 4.3.1 on?
Section 4.3 Quantitative chemistry is assessed on AQA GCSE Chemistry Paper 1, but the skills are used in calculations on Paper 2 too.
How do I explain a mass change in a reaction?
Decide whether a gas is a reactant (mass of the solid goes up as atoms are added from the air) or a product (mass goes down as gas escapes from an open container). Always state that mass is still conserved overall.
