JPJC 2026 Mole & Stoichiometry Tutorial

2026 JPJC JC1 H2 Chemistry (9476) 1 Topic 2: The Mole Concept and Stoichiometry Jurong Pioneer Junior College 2026 JC1 H2 Chemistry (9476) Tutorial 2: The Mole Concept and Stoichiometry Level 1 – Self–Check Questions 1 (a) Define the term relative atomic mass. (b) [2017 P1 Q1] & [2023 P1 Q8] A sample of silicon contains three naturally occurring isotopes, 28Si, 29Si and 30Si. The sample is made up of 92.23% 28Si and the relative atomic mass of silicon in this sample is 28.10. Calculate the percentage of the isotope 29Si in the sample. [5.54%] 2 Calculate the amount of the stated particles in each of the following cases. (a) F2 molecules in 10.0 g of fluorine, F2 (b) CaCO3 units in 250 g of calcium carbonate, CaCO3 (c) K+ ions in 9.03  1023 K2SO4 units (d) H atoms in 370 cm3 of ammonia, NH3, gas at standard temperature and pressure. [0.263 mol; 2.50 mol ; 3.00 mol; 0.0489 mol] 3 Calculate the number of the stated particles in each of the following cases. (a) O atoms in 320 g of oxygen, O2, gas (b) H atoms in 60 cm3 of methane, CH4, gas at room temperature and pressure (c) K+ ions in 27.5 cm3 of 0.120 mol dm–3 potassium sulfate, K2SO4, solution (d) electrons in 5 g of hydrogen, H2, gas [1.20  1025; 6.02  1021; 3.97  1021; 3.01  1024] 4 A solution is prepared by dissolving 1.86 g of potassium chloride, KCl, and 8.29 g of potassium carbonate, K2CO3, in enough water to make up the final volume to 500 cm3. Calculate the concentration of the potassium, K+, ions in the solution. [0.290 mol dm–3] 5 What volume of 0.0500 mol dm–3 potassium dichromate(VI), K2Cr2O7, solution should you transfer into a 250 cm3 volumetric flask in order to dilute the solution to 0.0200 mol dm–3? [100 cm3]

2026 JPJC JC1 H2 Chemistry (9476) 2 Topic 2: The Mole Concept and Stoichiometry 6 Phosgene, COCl2, was once used as a war gas. It is poisonous because when inhaled, it reacts with water in the lungs to produce carbon dioxide , CO2, gas and hydrochloric acid, HCl, which causes severe lung damage, leading to death ultimately. (a) Calculate the percentage by mass of chlorine in COCl2. (b) Write a balanced equation for the reaction between COCl2 and H2O. (c) Calculate the amount of HC l that will be produced by the complete reaction of 0.430 mol of COCl2. (d) Identify the limiting reagent when 0.200 mol of COC l2 is mixed with 6.20 g of H 2O. Hence, calculate the amount of HCl produced at the end of the reaction. [71.7%; 0.860 mol; 0.400 mol] 7 7.18 g of a sample of hydrated salt, Na2CO3.xH2O, was heated strongly until there was no further decrease in mass. Na2CO3.xH2O(s) → Na2CO3(s) + xH2O(g) If the remaining residue, Na2CO3 has a mass of 2.65 g, calculate the relative formula mass of the hydrated salt. Hence, determine the value of x. [x=10] 8 An organic c ompound R with molar mass 134 g mol–1 has the following composition by mass: C, 35.8 % H, 4.5 % O, 59.7 % Calculate the empirical formula of R and hence deduce its molecular formula. [C4H6O5; C4H6O5] 9 Styrene is a hydrocarbon (compound containing carbon and hydrogen only) that is used in the production of polystyrene. On combustion, 1.000 g of styrene produces 3.385 g of carbon dioxide and 0.692 g of water. Given the molar mass of styrene is 104 g mol –1, determine the empirical and molecular formulae of styrene. 10 After complete combustion of 100 cm 3 of hydrocarbon Y, 400 cm 3 of carbon dioxide and 500 cm3 of steam were formed. All volume measurements are done at 1 atm and 150 °C. Deduce the molecular formula of Y. [C4H10] 11 3.50 g of an impure sample of zinc was added to 200 cm 3 of 0.500 mol dm–3 hydrochloric acid. After effervescence had stopped, 25.0 cm 3 of the resulting solution required 21.10 cm3 of 0.0500 mol dm–3 sodium hydroxide for complete neutralisation. Calculate the percentage purity of the zinc sample. [85.5%]

2026 JPJC JC1 H2 Chemistry (9476) 3 Topic 2: The Mole Concept and Stoichiometry 12 What is the oxidation state of the named element in the following ions/compounds? (a) Cl in Cl2O7 (b) Cr in Na3CrO4 (c) O in BaO2 (d) I in ICl4– (e) C in C2O42– [+7; +5; –1; +3; +3] 13 Identify which of the following are redox reactions. Explain your answer in terms of change in oxidation numbers. (a) H2O2 + 2I– + 2H+ → 2H2O + I2 (b) KIO3 + 2KI + 6HCl → 3KCl + 3ICl + 3H2O (c) 2CrO42– + 2H+ → Cr2O72– + H2O (d) 2H2O2 → 2H2O + O2 Level 2 – Application (Discussion) Questions 14 In the blast furnace containing 500 tonnes of iron( III) oxide and 70 tonnes of coke, it was found that only 250 tonnes of iron were produced. 2Fe2O3(s) + 3C(s) → 3CO2(g) + 4Fe(s) iron(III) oxide coke Calculate the percentage yield of iron and determine t he theoretical volume of carbon dioxide released at room temperature and pressure. [1 tonne = 1000 kg] [71.5 %; 1.13  108 dm3] 15 20 cm 3 of a gaseous hydrocarbon A, C xHy, was exploded with an excess of oxygen. Upon cooling to room temperature, there was a contraction in volume of 50 cm3. When the products were treated with excess potassium hydroxide, there was a further contraction of 60 cm3. Deduce the molecular formula of A. All volumes were measured at r.t.p. [C3H6] 16 [2022 P2 Q3(f) – modified] Compound N has the formula CxHyO. It is a liquid at room temperature and pressure. Complete combustion of N occurs when 6.25  10–4 mol of N is burnt in a vessel which contains 100 cm3 of pure oxygen gas, measured at room temperature and pressure. When the mixture is cooled to room temperature and pressure, a total volume of 92.5 cm3 of gas remains. When this gas is passed repeatedly over NaOH(s), the final volume of gas which remains is 77.5 cm3. Determine the formula of compound N. [CH4O]

2026 JPJC JC1 H2 Chemistry (9476) 4 Topic 2: The Mole Concept and Stoichiometry 17 A 10.00 g of a sample of fertiliser which contained ammonium sulfate, (NH 4)2SO4 was boiled with 25.0 cm3 of 2.00 mol dm–3 sodium hydroxide. The ammonium sulfate released ammonia as shown by the following equation: (NH4)2SO4(s) + 2NaOH(aq) → Na2SO4(aq) + 2NH3(g) + 2H2O(l) After all the ammonia was driven off, the resulting solution was made up to 250 cm 3. 25.0 cm3 of this final solution was neutralised by 24.55 cm3 of 0.0500 mol dm–3 hydrochloric acid. Calculate the percentage by mass of ammonium sulfate in the fertiliser. [24.9 %] 18 [2021 P2 Q5(f)] The standard solution of iron( II) ions is prepared by making up a standard solution of Fe(NH4)2(SO4)26H2O. A sample of x g of Fe(NH 4)2(SO4)26H2O is weighed and transferred to a 250 cm 3 volumetric flask. It is dissolved in sulfuric acid and made up to the required volume using deionised water. 10.00 cm 3 of this solution is taken and transferred to a volumetric flask. Deionised water is added to make a total volume of 100.0 cm 3 of diluted solution. This diluted solution contains 10.0 ppm of iron ions. (1 ppm of iron(II) ions = 1 mg of iron(II) ions in 1000 cm3 of solvent.) [Mr Fe(NH4)2(SO4)26H2O = 392.0] Calculate the value of x in g. Show your working. [0.176 g] 19 Write balanced equations for the following reactions: (a) the reduction of Cr2O72–(aq) to Cr3+(aq) with S2O32–(aq) oxidised to S4O62–(aq) in acid solution. (b) the reduction of NO 3–(aq) to NH 3(aq) with Fe(OH) 2(s) oxidised to Fe(OH) 3(s) in alkaline solution. (c) the oxidation of Cr(OH)3(s) to CrO42–(aq) by H2O2(aq) in alkaline solution 20 (a) [N2017 P3 Q4(a)(i)(ii)] (i) Calculate the average oxidation number of carbon in butanoic acid,