2 Helium, neon, argon, krypton, xenon and radon are noble gases in Group VIII. (a) Name the noble gas which has the greatest volume composition in air. … [1] (b) State one use for helium. … [1] (c) Radon is very unreactive. Use the electronic structure of radon to explain why. … … [1] (d) Two isotopes of radon are shown. 220 86Rn 222 86Rn (i) Give one similarity in the atomic structure of these two isotopes. … [1] (ii) Give one difference in the atomic structure of these two isotopes. … [1] (e) Xenon forms a compound that contains only xenon, oxygen and fluorine. The compound contains 22.1% oxygen by mass and 17.5% fluorine by mass. Calculate the empirical formula of this compound. empirical formula … [3] (f) A sample of neon has a volume of 21 dm3 at room temperature and pressure. (i) The temperature of the sample is increased. The pressure remains constant. Describe and explain, using kinetic particle theory, what happens to the volume of the sample. … … [1] (ii) The pressure of the sample is increased. The temperature remains constant. Describe and explain, using kinetic particle theory, what happens to the volume of the sample. … … [1] (iii) Calculate the mass of neon in the 21 dm3 sample. Give your answer to two significant figures. mass … g [2] [Total: 12]

2(a) argon 1 2(b) (air) balloons / undersea breathing apparatus 1 2(c) outer shell has 8 electrons 1 2(d)(i) 86 protons / 86 electrons / same electronic structure 1 2(d)(ii) radon-222 has two more neutrons / radon-220 has two fewer neutrons / radon-220 has 134 neutrons and radon-222 has 136 neutrons 1 2(e) percentage of xenon = 60.4 (1) mole ratio Xe : O : F is 60.4 / 130 : 22.1 / 16 : 17.5 / 19 OR 0.46 : 1.38 : 0.92 (1) empirical formula XeO3F2 (1) 3 2(f)(i) volume increases AND particles move faster / particles spread out more / particles move further apart 1 2(f)(ii) volume decreases AND particles move closer together / distance between particles decreases 1 2(f)(iii) moles = 21 / 24 OR 0.875 (1) mass = 18 (g) (1) 2 To earn marks, you must correctly name argon, state a valid use for helium, explain radon's unreactivity by mentioning its full outer electron shell, identify a similarity (protons/electrons/electronic structure) and a difference (number of neutrons) between the isotopes, and correctly calculate the empirical formula by showing the percentage of xenon, the mole ratios, and the final formula. For parts (f)(i) and (f)(ii), you need to state the change in volume and explain it in terms of particle movement. For (f)(iii), you must correctly calculate the moles and then the mass.

2 Oxygen, sulfur, selenium, tellurium and polonium are in Group VI. (a) State the percentage composition by volume of oxygen in dry air. … [1] (b) State one large-scale use for oxygen. … [1] (c) Two isotopes of polonium are shown. 20 8 9 4Po 21 8 0 4Po (i) Explain why both isotopes have the same chemical properties. … [1] (ii) Give one difference in the atomic structure of these two isotopes. … [1] (d) Selenium forms a compound that contains only selenium, oxygen and chlorine. The compound contains 9.6% oxygen by mass and 42.8% chlorine by mass. Calculate the empirical formula of this compound. empirical formula … [3] (e) A sample of oxygen has a volume of 11.5 dm3 at room temperature and pressure. (i) The temperature of the sample is decreased. The pressure remains constant. Describe and explain, using kinetic particle theory, what happens to the volume of the sample. … … [1] (ii) The pressure of the sample is decreased. The temperature remains constant. Describe and explain, using kinetic particle theory, what happens to the volume of the sample. … … [1] (iii) Calculate the mass of oxygen in the 11.5 dm3 sample at room temperature and pressure. Give your answer to two significant figures. mass … g [2] [Total: 11]

2(a) 21 (%) 1 2(b) steel making / welding / cutting metals / oxygen tents in hospitals / breathing apparatus / fuel cells 1 2(c)(i) same electronic structure / same electronic configuration 1 2(c)(ii) polonium-210 has one more neutron / polonium-209 has one less neutron / polonium-209 has 125 neutrons and polonium- 210 has 126 neutrons 1 2(d) percentage of selenium = 47.6 (1) mole ratio Se : O : Cl is 47.6 / 79 : 9.6 / 16 : 42.8 / 35.5 OR 0.60 : 0.60 : 1.21 (1) empirical formula SeOCl 2 (1) 3 2(e)(i) volume decreases AND particles move slower / particles less spread out / particles closer together 1 2(e)(ii) volume increases AND particles move further apart / particles more spread out / distance between particles increases 1 2(e)(iii) mol = 11.5 / 24 OR 0.479(117) (1) mass = 15 (g) (1) 2 To earn marks, you must provide the correct percentage for oxygen in air, state a valid large-scale use for oxygen, explain that isotopes have the same chemical properties due to identical electronic structures, and identify the difference in neutron count between the given isotopes. For the empirical formula calculation, you need to correctly calculate the percentage of selenium, determine the mole ratios, and then state the correct empirical formula. For part (e)(i), you must state that the volume decreases and explain this using kinetic particle theory by mentioning particles moving slower or being closer together. For part (e)(ii), you must state that the volume increases and explain this by mentioning particles moving further apart or being more spread out. For part (e)(iii), you must correctly calculate the moles of oxygen and then use this to find the mass.

3 Iron powder reacts with dilute sulfuric acid. Fe(s) + H2SO4(aq) FeSO4(aq) + H2(g) (a) The graph shows the volume of hydrogen gas produced at 20 °C as the reaction proceeds. volume of hydrogen gas / cm3 40 30 20 10 0 4 8 12 16 20 24 28 time / min (i) State how long it takes from the start of the experiment to collect 18 cm3 of hydrogen gas. … [1] (ii) The experiment is repeated at 30 °C. All other conditions are the same. Draw a line on the grid to show how the volume of hydrogen gas produced changes with time when the reaction is done at 30 °C. [2] (b) (i) Describe and explain, using ideas about collisions between particles, how the rate of the reaction changes when the temperature of the reaction mixture is increased. All other conditions are the same. … … … … [2] (ii) Describe and explain, using ideas about collisions between particles, how the rate of the reaction changes when larger pieces of iron are used. All other conditions are the same. … … … … [2] (c) Calculate the maximum volume, in dm3, of hydrogen formed when 3.36 g of iron react with excess dilute sulfuric acid at room temperature and pressure. Give your answer to three significant figures. maximum volume = … dm3 [2] (d) Concentrated sulfuric acid is an oxidising agent. Describe a test for oxidising agents. test … observations … … [2] [Total: 11]

3(a)(i) 6 minutes 1 3(a)(ii) initial gradient steeper and starting from origin (1) line levels off at 34 cm3 gas (1) 2 3(b)(i) rate of reaction increases (no mark) and particles move faster / particles have more kinetic energy (1) more particles have activation energy (or above) / more successful collisions / more energetic collisions (1) 2 3(b)(ii) rate of reaction decreases (no mark) and fewer particles exposed on surface / fewer particles (of iron) per unit area / fewer particles per cm2 (1) collision frequency decreases / fewer particles collide per second / collision rate decreases / collisions less often (1) 2 3(c) mol iron = 3.36 56 OR 0.060 mol (1) volume of hydrogen = 1.44 (dm3) (1) 2 3(d) add (aqueous) potassium iodide (1) colourless to brown solution (1) 2 (a)(i) To earn the mark, you must correctly read the graph and state the time as 6 minutes. (a)(ii) To earn the first mark, you must draw a line starting from the origin with a steeper initial gradient than the original line. To earn the second mark, your drawn line must level off at 34 cm3 of gas. (b)(i) To earn the first mark, you must state that particles move faster or have more kinetic energy. To earn the second mark, you must explain that more particles have activation energy (or above), leading to more successful collisions or more energetic collisions. (b)(ii) To earn the first mark, you must state that fewer particles are exposed on the surface, or fewer particles of iron are present per unit area/cm². To earn the second mark, you must explain that collision frequency decreases, or fewer particles collide per second, or the collision rate decreases. (c) To earn the first mark, you must correctly calculate the moles of iron (3.36/56 or 0.060 mol). To earn the second mark, you must correctly calculate the volume of hydrogen as 1.44 dm3. (d) To earn the first mark, you must state adding aqueous potassium iodide. To earn the second mark, you must state the observation of a colourless to brown solution.

3 Magnesium ribbon reacts with dilute hydrochloric acid. Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g) (a) The graph shows the volume of hydrogen gas produced at 20 °C as the reaction proceeds. The magnesium is in excess. volume of hydrogen gas / cm3 60 50 40 30 20 10 0 1 2 3 4 5 6 7 time / min The experiment is repeated using a lower concentration of hydrochloric acid. The volume of acid used and all other conditions are the same. Draw a line on the grid to show how the volume of hydrogen gas produced changes with time when the reaction is done with a lower concentration of hydrochloric acid. [2] (b) (i) Describe and explain, using ideas about collisions between particles, how the rate of the reaction changes when magnesium powder is used instead of magnesium ribbon. All other conditions are the same. … … … … [2] (ii) Describe and explain, using ideas about collisions between particles, how the rate of the reaction changes when the temperature of the reaction mixture is decreased. All other conditions are the same. … … … … [2] (c) Calculate the maximum volume, in dm3, of hydrogen formed when 1.68 g of magnesium react with excess dilute hydrochloric acid at room temperature and pressure. Give your answer to three significant figures. volume = … dm3 [2] (d) Magnesium is a good reducing agent. Describe a test for reducing agents. test … observations … … [2] [Total: 10]

3(a) initial gradient less steep AND starting from origin (1) line levels off at lower than 45 cm3 gas (1) 2 3(b)(i) rate of reaction increases AND one or both of: more particles exposed on surface / more (magnesium) particles per unit area / more particles per cm2 (1) collision frequency increases / collision rate increases / more collisions per second (1) 2 3(b)(ii) rate of reaction decreases AND one or both of: particles move more slowly / particles have less kinetic energy (1) fewer particles have activation energy (or above) / collisions are less successful (1) 2 3(c) mol magnesium = 1.68 24 OR 0.070 mol (1) volume of oxygen = 1.68 (dm3) (1) 2 3(d) add (aqueous acidified) potassium manganate(VII) (1) turns from purple to colourless (1) 2 (a) To earn the first mark, your drawn line must start at the origin and have a less steep initial gradient than the original line. To earn the second mark, your line must level off at a volume of hydrogen gas lower than 45 cm³. (b)(i) To earn the first mark, you must state that the rate of reaction increases. To earn the second mark, you must explain this by mentioning more particles exposed on the surface or more particles per unit area, leading to an increased collision frequency. (b)(ii) To earn the first mark, you must state that the rate of reaction decreases. To earn the second mark, you must explain this by mentioning that particles move more slowly or have less kinetic energy, resulting in fewer particles having the activation energy or collisions being less successful. (c) To earn the first mark, you must correctly calculate the moles of magnesium as 0.070 mol (or 1.68/24). To earn the second mark, you must correctly state the volume of oxygen as 1.68 dm³. (d) To earn the first mark, you must state that aqueous acidified potassium manganate(VII) is added. To earn the second mark, you must state that the solution turns from purple to colourless.

9 The empirical formula of compound X is CH2 and the relative molecular mass, Mr, of X is 70. What is the molecular formula of X? A CH2 B C2H4 C C5H10 D CnH2n

Answer: C To earn the mark, the student must correctly determine the molecular formula (C5H10) by finding the multiple of the empirical formula (CH2) that gives a relative molecular mass of 70.

Cambridge O Level Chemistry · 5070

O-Level Chemistry (5070) Stoichiometry past paper questions

Revise O-Level Chemistry (5070) — Stoichiometry with real Cambridge past-paper questions grouped by topic, each shown with its marking scheme. Working one topic at a time targets exactly the skill each question tests.

234 free questions from 2016–2021, each with its marking scheme. Open the full topical bank to filter by subtopic, paper, and year.

Stoichiometry in O-Level Chemistry 5070 exams, 2020–2025

Across 2020–2025, Stoichiometry accounted for 237 questions worth 1,555 marks in Cambridge O Level Chemistry (5070) papers.

By year
Year	Questions	Marks
2020	39	308
2021	40	321
2022	43	280
2023	35	190
2024	45	216
2025	35	240

Most-tested subtopics

3.3 The mole and the Avogadro constant1061 marks
3.1 Formulae958 marks
3.2 Relative masses of atoms and molecules96 marks

Appears in

Paper 1 · 115 qs
Paper 2 · 80 qs
Paper 3 · 19 qs
Paper 4 · 23 qs

All most-tested Chemistry topics

2021 May/Jun · Paper 2 · Q2 · 12 marks
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2021 May/Jun · Paper 2 · Q2 · 11 marks
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2021 Oct/Nov · Paper 2 · Q3 · 11 marks
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2021 Oct/Nov · Paper 2 · Q3 · 10 marks
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2021 May/Jun · Paper 1 · Q9 · 1 mark
Marking scheme
Mark scheme — 2021 Paper 1 Q9
Answer: C
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