JPJC 2026 The Mole Concept & Stoichiometry Revision

1 2026 JC1 H2 Chemistry (9476) Topical Revision for Physical Chemistry (Topic 2: The Mole Concept & Stoichiometry) FOUNDATION KIT MCQs 1 [PJC MYE 2013/P1/Q1] Naturally occurring silicon is a mixture of three isotopes, 28Si, 29Si and 30Si. The relative atomic mass of silicon is 28.109. What could be the relative abundance of each of the three isotopes? A 91.1% 28Si, 7.9% 29Si and 1.0% 30Si B 92.2% 28Si, 4.7% 29Si and 3.1% 30Si C 95.0% 28Si, 3.2% 29Si and 1.8% 30Si D 96.3% 28Si, 0.3% 29Si and 3.4% 30Si 2 [ACJC Prelim2020/P1/Q2] Use of the Data Booklet is relevant to this question. The approximate percentage composition of the atmospheres on four planets are given below. The density of a gas may be defined as the mass of 1 dm3 of the gas. Which mixture of gases has the greatest density? planet gases / % by volume A Jupiter H2 88.8, He 10.2, Ar 1.0 B Neptune H2 80.0, He 19.0, CH4 1.0 C Saturn H2 92.3, He 7.2, CH4 0.5 D Uranus H2 82.5, He 15.2, CH4 2.3 3 [PJC MYE 2013/P1/Q3] Use of the Data Booklet is relevant to this question. Carbon monoxide, CO, is a colourless, odourless and toxic gas. It is formed as a result of partial oxidation of carbon −containing compounds. The maximum safe tolerant level of CO in air is 50 ppm. (1 ppm = 1 mg of substance / 1 kg of air) How many CO molecules are present in 1 kg of air at this maximum safe tolerant level? A 50  103  28  6.02  1023 B 50  10−3  28  6.02  1023 C   3 2350 10 6.02 10 28 D −  3 2350 10 6.02 10 28

2 4 [IJC MYE 2011/P1/Q1] Use of the Data Booklet is relevant to this question. Which of the following contains the same number of atoms as 16 g of oxygen gas, O2? A 17 g of ammonia, NH3 B 2 g of helium, He C 8 g of methane, CH4 D 14 g of nitrogen gas, N2 5 [ACJC Prelim2020/P1/Q1] In July 2020, some households in Singapore reported a Pandan smell in their tap water. This was caused by an organic solvent tetrahydrofuran, THF, which has the following structure. THF (molecular formula: C4H8O) The laboratory found that the concentration of THF in the Pandan smelling water was less than 10 ppb (10 parts per billion) by mass . This means that there was less than 10 g of THF in 109 g of water, which was significantly lower than the levels of THF that would cause health concerns. What is the concentration of 10 ppb of THF in mol dm−3? Given: density of water = 1.0 g cm−3 A 1.39  10−10 B 1.39  10−7 C 1.56  10−7 D 6.40  10−4 6 In which compounds does sulfur exhibit its highest oxidation state? A S8 B SO2 C SO2Cl2 D Na2S2O3

3 7 [PJC MYE 2012/P1/Q1] For complete oxidation, 1 mol of an organic compound requires 3 mol of oxygen gas. What could be the formula of the compound? A CH3CHO B CH3CH2OH C CH3CH3 D CH3CO2H 8 [PJC MYE 2012/P1/Q3] Use of the Data Booklet is relevant to this question. How many molecules are present in 1 cm3 of oxygen gas under room condition? A 23 1 24000 6.02 10   B 231 6.02 10 24000  C 231 6.02 10 32   D 236.02 10 24000 1 1000   9 [PJC MYE 2011/P1/Q2] Carbon disulfide, CS2, is a volatile flammable liquid used in the manufacture of cellophane. On combustion, CS2 is oxidised as follows. CS2(g) + 3O2(g) → CO2(g) + 2SO2(g) A 20 cm 3 sample of carbon disulfide vapour is ignited with 100 cm 3 of oxygen. The final volume of gas after burning is treated with an excess of aqueous alkali. Which percentage of this final volume dissolves in the alkali? [All volumes are measured at the same temperature and pressure, conditions under which, CS2 is a gas.] A 20 % B 40 % C 60 % D 80 % 10 [JJC Prelim 2018/P1/Q5 - modified] 10 cm 3 of a hydrocarbon was exploded in excess oxygen. Upon cooling to room temperature, there was a contraction in volume of 35 cm 3. When the products were treated with excess KOH(aq), there was a further contraction of 40 cm3. Deduce the molecular formula of the hydrocarbon. All volumes were measured at r.t.p. A C2H4 B C4H6 C C4H8 D C4H10

4 11 [PJC Prelim 2014/P1/Q2] Use of the Data Booklet is relevant to this question. Sodium percarbonate, (Na 2CO3)x•y(H2O2), is an oxidising agent in some home and laundry cleaning products. 5.0 cm3 of 0.200 mol dm −3 sodium percarbonate releases 48.0 cm 3 of carbon dioxide at room conditions on reaction with excess hydrochloric acid. An identical sample, on titration with 0.100 mol dm −3 acidified KMnO4, requires 12.0 cm3 before the first pink colour appears. What is the ratio x : y? [Hint: Write a balanced equation for the reaction of Na 2CO3 with hydrochloric acid, and write a balanced equation for the reaction of MnO4– with H2O2.] A 1 : 3 B 2 : 3 C 3 : 2 D 4 : 3 12 [PJC Promo 2013/P1/Q2] Use of the Data Booklet is relevant to this question. Hydrogen cyanide, HCN, was used by Nazi Germany during the Holocaust in gas chambers for mass murder. A hydrogen cyanide concentration of 300 mg m−3 is sufficient to kill a person within 1 hour. How many molecules of hydrogen cyanide are present in 1 dm 3 of air at the stated concentration? A 3 3 23 300 10 27 10 6 10 −    B 3 23 3 300 10 6 10 27 10 −    C 3 3 23 300 10 27 10 6 10 −  D 3 23 3 27 10 6 10 300 10 −    13 [PJC Promo 2014/P1/Q2] To identify an oxide of nitrogen, 0.1 mol of the oxide is mixed with an excess of hydrogen and passed over a catalyst at a suitable temperature. NxOy(g) + 3 + 2 2 xy H2(g) x NH3(g) + y H2O(l) The water produced weighs 7.20 g. The ammonia produced is neutralised by 200 cm 3 of 1.0 mol dm-3 HCl. What is the formula of the oxide of nitrogen? A N2O B NO C NO2 D N2O4

5 14 [JJC Prelim 2012/P1/Q1] Use of the Data Booklet is relevant to this question. When x mol of cobalt is added to a solution containing y mol of cobalt(III) ions, cobalt(II) ions are formed in the resultant solution. In the resultant solution, the number of moles of cobalt( II) ions are three times that of cobalt(III) ions. Which of the following are possible values of x and y? x y A 1 2 B 1 3 C 1 5 D 2 3 15 [RVHS Prelim 2012/P1/Q2] An ion of metal M can be oxidised by acidified potassium manganate(VII) to form MO3−. In an experiment, 1.25  10−3 mol of the ion of M required 37.5 cm 3 of 0.0200 mol dm −3 potassium manganate(VII) for complete reaction. What is the initial oxidation state of the ion of M? A +1 B +2 C +3 D +4 16 [PJC Promo 2016/P1/Q3] 25.0 cm 3 of 0.05 mol dm −3 KClO4(aq) required 50.0 cm 3 of 0.20 mol dm −3 TiCl3 (aq) to reach end−point. Given that titanium( III) is oxidised to titanium( IV) in this reaction, which of the following formulae correctly represents the reduction product of the ClO4− ion? A Cl− B ClO2− C ClO3− D ClO− MCQ Answer keys: 1 2 3 4 5 6 7 8 B D D D B C B B 9 10 11 12 13 14 15 16 C D B B D B B A

6 Structured & Free-Response Questions 17 [VJC Prelim 2011/P2/Q2c – modified] 0.500 g of a Group 2 iodate(V) salt, M(IO3)2, was heated and decomposed to give metal oxide, MO, purple iodine gas and colourless oxygen gas. (a) Write a balanced equation for the decomposition reaction, including state symbols. (b) Given that 0.0532 g of the white solid was collected, calculate the relative atomic mass of the metal, M, and hence deduce its identity. [23.8] 18 [JPJC 2018 Promo/P2/Q16] 3.000 g of impure ammonium sulfate, (NH 4)2SO4 (Mr = 132.1), was boiled with 50.0 cm 3 of 1.00 mol dm−3 sodium hydroxide solution until no more ammonia gas was evolved. (NH4)2SO4 + 2NaOH → Na2SO4 + 2NH3 + 2H2O The resulting solution was made up to 100 cm3 with deionised water. 25.0 cm3 of this diluted solution required 27.00 cm 3 of 0.0450 mol dm –3 sulfuric acid for neutralisation. (a) Calculate the amount of sodium hydroxide reacted with the sulfuric ac