2 The table shows the mass of some of the ions in a 1000 cm3 sample of sea water. name of ion formula of ion mass of ion in 1000 cm3 of sea water / mg bromide Br – 65 calcium Ca2+ 400 chloride Cl – 18 980 hydrogencarbonate HCO3 – 140 magnesium Mg2+ 1262 metaborate B3O6 3– 26 K+ 380 sodium Na+ 10 556 strontium Sr2+ 13 SO4 2– 2649 (a) Answer these questions using only the information in the table. (i) State which negative ion has the lowest mass in 1000 cm3 of sea water. … [1] (ii) Give the formulae of the ions in potassium sulfate. … and … [1] (iii) Calculate the mass of calcium ions in 200 cm3 of this sample of sea water. mass = … mg [1] (iv) A sample of this sea water is evaporated. State the name of the compound which is present in the greatest quantity when this sample is evaporated. … [1] (v) Give the name of the ion which reacts with aqueous silver nitrate to give a cream precipitate. … [1] (b) The B3O6 3– ion can be converted to boric acid, H3BO3. Boric acid is also produced when boron trichloride, BCl 3, reacts with water. Complete the equation for this reaction. BCl 3 + …H2O → H3BO3 + …HCl [2] (c) The symbol of a strontium ion is shown. 87Sr2+ 38 Deduce the number of electrons, protons and neutrons in one atom of this strontium ion. number of electrons … number of protons … number of neutrons … [3] (d) Some isotopes of strontium are radioactive. (i) Give one medical use of radioactive isotopes. … [1] (ii) The isotope 235U is also radioactive. State the major use of this isotope of uranium. … [1] [Total: 12]

2(a)(i) B3O63– / metaborate 1 2(a)(ii) K+ AND SO42– 1 2(a)(iii) 80 (mg) 1 2(a)(iv) sodium chloride 1 2(a)(v) bromide 1 2(b) 3 (H2O) (1) 3 (HCl) (1) 2 2(c) number of electrons: 36 (1) number of protons: 38 (1) number of neutrons: 49 (1) 3 2(d)(i) cancer treatment / tracer (e.g. for thyroid function) 1 2(d)(ii) source of energy / energy production 1 To earn marks, you must correctly identify the specified ion, provide the correct formulae for the ions in potassium sulfate, calculate the correct mass of calcium ions, name the compound present in the greatest quantity after evaporation, and name the ion that forms a cream precipitate with aqueous silver nitrate.

2 The table shows the masses of some of the ions in 1000 cm3 of fruit juice. name of ion formula of ion mass of ion in 1000 cm3 of fruit juice / mg NH4 + 43 calcium Ca2+ 79 chloride Cl – 135 lithium Li+ 1 magnesium Mg2+ 80 nitrate NO3 – 35 phosphate PO4 3– 120 potassium K+ 575 sodium Na+ 120 SO4 2– 105 (a) Answer these questions using only the information in the table. (i) State which negative ion has the highest mass in 1000 cm3 of fruit juice. … [1] (ii) Give the formulae of the ions in ammonium sulfate. … and … [1] (iii) Calculate the mass of sodium ions in 200 cm3 of fruit juice. mass = … mg [1] (b) Describe a test for lithium ions. test … observations … [2] (c) Ions of the element potassium, K, are present in most fertilisers. State the names of two other elements that are in most fertilisers. 1 … 2 … [2] (d) Orange juice is acidic. Draw a circle around the pH of orange juice. pH 4 pH 7 pH 10 pH 13 [1] (e) Some soils are acidic. Give the names of two compounds that are used to make soils less acidic. 1 … 2 … [2] (f) Hydrogen chloride is an acidic gas produced when concentrated hydrochloric acid evaporates. (i) Describe the arrangement and separation of the molecules in hydrogen chloride gas. arrangement … … separation … … [2] (ii) A long glass tube is set up as shown. long glass tube cotton wool soaked in concentrated hydrochloric acid damp blue litmus paper At first, the blue litmus paper does not turn red. After a short time, the litmus paper turns red. Explain these observations using the kinetic particle model. … … … … … [3] [Total: 15]

2(a)(i) Cl – / chloride 1 2(a)(ii) NH4+ AND SO42- 1 2(a)(iii) 24 (mg) 1 2(b) flame test / description of flame test (1) (flame coloured) red (1) 2 2(c) nitrogen (1) phosphorus (1) 2 pH 4 circled 1 2(e) 1 mark each for any 2 of: • calcium carbonate • calcium oxide • calcium hydroxide 2 2(f)(i) arrangement: random (arrangement) / no fixed arrangement / no pattern / no fixed position (1) separation: far apart / far away (from each other) / distant (from each other) (1) 2 2(f)(ii) 1 mark each for any 3 of: • (HCl) molecules escape from solution • diffusion • molecules in (constant) movement / molecules collide / molecules travel • (movement of) molecules is random / haphazard / in every direction • molecules spread out / molecules mix • (molecules spread) from high(er) concentration to low(er) concentration • molecules hit the litmus paper / molecules 3 (a) To earn marks, you must identify the correct negative ion, provide both correct ion formulae, and calculate the correct mass of sodium ions. (b) To earn marks, you must describe a flame test and state that the observation is a red flame. (c) To earn marks, you must state nitrogen and phosphorus as the two other elements.

2 The table shows the masses of some of the ions in 1000 cm3 of fruit juice. name of ion formula of ion mass of ion in 1000 cm3 of fruit juice / mg ammonium NH4 + 15 Ca2+ 71 chloride Cl – 135 magnesium Mg2+ 160 nitrate NO3 – 2 phosphate PO4 3– 63 potassium K+ 184 sodium Na+ 3 SO4 2– 85 (a) Answer these questions using only the information in the table. (i) State which positive ion has the lowest mass in 1000 cm3 of fruit juice. … [1] (ii) Give the formulae of the ions in calcium sulfate. … and … [1] (iii) Calculate the mass of magnesium ions in 250 cm3 of fruit juice. mass = … mg [1] (b) Describe a test for calcium ions. test … observations … [2] (c) Ammonium ions, NH4 +, are present in most fertilisers. Ammonium ions contain nitrogen. Name two other elements present in most fertilisers. 1 … 2 … [2] (d) A student heated a mixture of ammonium chloride and sodium hydroxide in a test‑tube. NH4Cl + NaOH → NH3 + NaCl + H2O mixture of ammonium chloride and sodium hydroxide heat Pungent-smelling ammonia gas is given off. Describe one other observation that can be made. … [1] (e) Ammonia reacts with chlorine. Complete the equation for this reaction. …NH3 + …Cl 2 → N2 + 6HCl [2] (f) A small beaker of aqueous ammonia is placed at the front of a classroom. At first, the students at the back of the class do not smell the ammonia gas. After a short time, the students at the back of the class smell the ammonia. Explain these observations using the kinetic particle model. … … … … … [3] [Total: 13]

2(a)(i) Na + / sodium 1 2(a)(ii) Ca2+ AND SO42- 1 2(a)(iii) 40 (mg) 1 2(b) (add aqueous) sodium hydroxide (1) white precipitate (1) OR ( add aqueous) ammonia (1) no precipitate / very slight white precipitate (1) 2 2(c) potassium (1) phosphorus (1) 2 2(d) colourless liquid (condenses at the mouth of the tube) 1 2(e) 2 (NH3) (1) 3 (Cl2) (1) 2 2(f) 1 mark each for any 3 of: • evaporation of ammonia molecules (from the solution) / (ammonia) molecules escape from solution • diffusion • molecules in (constant) movement / molecules collide / molecules travel • (movement of) molecules is random / in every direction / haphazard • molecules spread out / molecules mix • (smell occurs when) molecules hit (the sensory cells in) the nose • (molecules spread) from higher concentration to lower concentration 3 To earn marks, you must correctly identify the positive ion with the lowest mass and provide the correct formulae for calcium and sulfate ions in part (a). For part (a)(iii), calculate the mass of magnesium ions in 250 cm3. In part (b), describe a valid test for calcium ions (e.g., adding sodium hydroxide or ammonia) and state the correct observation. For part (c), name two other elements found in most fertilisers besides nitrogen.

2 The table shows the masses of some of the ions in 1000 cm3 of fruit juice. name of ion formula of ion mass of ion in 1000 cm3 of fruit juice / mg ammonium NH4 + 6 calcium Ca2+ 73 chloride Cl – 238 magnesium Mg2+ 77 NO3 – 10 phosphate PO4 3– 20 potassium K+ 419 Na+ 3 sulfate SO4 2– 10 (a) Answer these questions using only the information in the table. (i) State which negative ion has the highest mass in 1000 cm3 of fruit juice. … [1] (ii) Give the formulae of the ions in sodium nitrate. … and … [1] (iii) Calculate the mass of ammonium ions in 250 cm3 of fruit juice. mass = … mg [1] (b) Describe a test for chloride ions. test … observations … [2] (c) Phosphate ions, PO4 3–, are present in most fertilisers. Phosphate ions contain phosphorus. (i) State the names of two other elements that are in most fertilisers. 1 … 2 … [2] (ii) Explain why farmers put fertilisers on fields where crops are to be grown. … … [1] (d) A student heated ammonium sulfate with sodium hydroxide in a test-tube. (i) Complete the equation for this reaction. (NH4)2SO4 + …NaOH → …NH3 + Na2SO4 + 2H2O [2] (ii) Concentrated aqueous ammonia releases fumes of ammonia gas. A long glass tube is set up as shown. long glass tube cotton wool soaked in concentrated aqueous ammonia damp red litmus paper At first, the red litmus paper does not turn blue. After a short time, the litmus paper turns blue. Explain these observations using the kinetic particle model. … … … … … [3] [Total: 13]

2(a)(i) Cl – / chloride 1 2(a)(ii) Na+ AND NO3– 1 2(a)(iii) 1.5 (mg) 1 2(b) add (aqueous acidified) silver nitrate (1) white precipitate / white solid (1) 2 2(c)(i) potassium (1) nitrogen (1) 2 2(c)(ii) to improve crop yield / to replace nitrogen (or other elements) lost from soil (by being absorbed by plants) / to replace nitrogen (or other elements) absorbed by plants 1 2(d)(i) 2 (NaOH) (1) 2 (NH3) (1) 2 2(d)(ii) 1 mark each for any 3 of: • (NH3) molecules escape from solution • diffusion • molecules in (constant) movement / molecules collide / molecules travel • (movement of) molecules is random / haphazard / in every direction • molecules spread out / molecules mix • (molecules spread) from high(er) concentration to low(er) concentration • molecules hit the litmus paper / molecules 3 (a) To earn marks, you must correctly identify the negative ion with the highest mass, provide the correct formulae for the ions in sodium nitrate, and calculate the correct mass of ammonium ions in 250 cm³ of fruit juice. (b) To earn marks, you must describe adding acidified silver nitrate solution and state that a white precipitate is observed. (c) To earn marks, you must name two other elements found in most fertilisers and explain why fertilisers are used.

Cambridge IGCSE Chemistry · 0620

IGCSE Chemistry (0620) Stoichiometry past paper questions

Stoichiometry questions test formulae, the mole, and reacting-mass and concentration calculations — the arithmetic core of the paper.

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

Stoichiometry in IGCSE Chemistry 0620 exams, 2020–2025

Across 2020–2025, Stoichiometry accounted for 416 questions worth 2,623 marks in Cambridge IGCSE Chemistry (0620) papers.

By year
Year	Questions	Marks
2020	69	424
2021	59	327
2022	70	443
2023	75	497
2024	70	477
2025	73	455

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2021 Feb/Mar · Paper 3 · Q2 · 12 marks
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2021 May/Jun · Paper 3 · Q2 · 15 marks
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2021 May/Jun · Paper 3 · Q2 · 13 marks
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2021 May/Jun · Paper 3 · Q2 · 13 marks
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2021 May/Jun · Paper 1 · Q6 · 1 mark
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Answer: A
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