Copy of CHIJ 2024 Sec 4E Chemistry Prelim P3 MS

CHIJ SECONDARY Sec 3 Chemistry EOY Exam 6092_2023 Mark Scheme / P 2/3 CHIJ SECONDARY Sec 4 Preliminary Examination 2024 Chemistry 6092 Mark Scheme Updated on 05/09/2024 Paper 3 (40 marks) Answers Marks Total 1 (a) (i) Results • correct headings and units [1] • burette readings to 2 d.p. [1] • accuracy within ±0.2 cm3 [2] or ±0.3 cm3 [1] of actual titre values • concordance within ±0.1 cm3 [1] 5 18 (ii) Calculation of average volume using best 2 values to 2 d.p. 1 (b) Volume of Q = X cm3 No. of moles of Q = X/1000 × 0.100 = 0.0001X mol [1] No. of moles of of K2CO3 in 25 cm3 = 0.0001X ÷ 2 = 0.00005X mol [1] 2 (c) No. of moles of K2CO3 in 1 dm3 = 1000/25 × 0.00005X mol = 0.002X mol 1 (d) (i) Mass concentration of of K2CO3 = 0.002X mol × 138 = 0.276X g/dm3 1 (ii) Mass of H2O in 1dm3 = 8.00 – 0.276X g 1 (iii) No. of moles of H2O = (8.00 – 0.276X) g ÷ 18 1 (iv) x = 0.09178 ÷ 0.046 ≈ 2 1 (e) Every 2 points is 1 mark: 1. Measure the mass [reject weight, penalize once] of an empty test-tube/ boiling tube/ evaporating dish/ crucible [reject beaker/ conical flask etc.] (p) using an electronic mass balance [reject weighing scale] 2. Add the solid sample to the test-tube, measure the initial mass of the test-tube and its contents (q) [accept measure the mass of hydrated K2CO3, if no apparatus mentioned point 1 not awarded] [reject measuring mass of solution] 3. Calculate the mass of hydrated K2CO3 used by taking q – p 4. Heat the sample (at regular / 1-minute intervals) [point only awarded if appropriate apparatus for heating was mentioned earlier; accept evaporate (to dryness)] 5. until the mass of the remaining solid / residue is constant / to dryness / all the water has evaporated [reject to saturation] 6. Measure the mass of the remaining residue and test-tube (r) [reject measure the mass of the test-tube after heating] [accept 5

CHIJ SECONDARY Sec 3 Chemistry EOY Exam 6092_2023 Mark Scheme / P 2/3 Answers Marks Total measure the mass of anhydrous K2CO3 as ecf if point 1 not mentioned] 7. Calculate the mass of the residue r – p [reject p – r] 8. Calculate the number of moles of K2CO3 by taking (r – p)/138. 9. Calculate the number of moles of H2O by taking (q – r)/18. 10. The value is x is the number of moles of H2O ÷ number of moles of K2CO3. OR 1. Using a burette/measuring cylinder/pipette, measure out 25.0 cm3 of solution P 2. into a conical flask. Set up using rubber stopper with delivery tube, connected to gas syringe [award point if set-up is drawn and labelled] 3. Add solution Q / HCl 4. in excess 5. Measure the volume of gas [reject amount/mass] 6. Calculate the number of moles of CO2 by taking v/24000 [reject finding no. of moles by mass/Mr if never mention to measure the mass of gas collected using electronic mass balance] 7. The number of moles of K2CO3 is also v/24000 8. The value of x can be determined using steps (c) to (d)(ii) (Max. 4 marks, because the method is not very different from the original question) 2 (a) A lilac/purple/violet flame was observed. 1 9 (b) The white solid melted to form a colourless liquid / water droplets were observed on the sides of the test-tube / a white residue remains [1] A gas was evolved that relights a glowing splint. [1] 2 (c) Test 1 • Upon adding potassium iodide, the yellow solution turns yellow/brown/reddish-brown/dark orange. [1] • Upon adding starch, a blue-black solution forms. [1] Test 2 • A white precipitate forms. [1] Test 3 • No visible change observed. [1] 4 (d) Nature: Oxidising agent (no mark) Reasoning: In Test 1, when potassium iodide is added, the solution changes to brown, [1] indicating that Y oxidised potassium iodide to form iodine. [1] 2 3 (a) Table • temperatures recorded to 1 d.p. [1] • correct trend: temperature decrease from t=2 to t=5.5 [1] (accept if first 2 temperatures are the same) • accuracy: temperature within 1°C difference at t=0, t=0.5 and t=1, and 66.0±3.0°C for highest temperature reading [1] 3 13 (b) Graph • correct axes labels and units, including scale [1] • temperature to 1 d.p., time to 1 d.p. or nearest whole number [1] • correctly plotted values [1] • best-fit lines [1] 4

CHIJ SECONDARY Sec 3 Chemistry EOY Exam 6092_2023 Mark Scheme / P 2/3 Answers Marks Total (c) Vertical line correctly drawn at 1.5 min [1] Temperature rise = 67.5 – 30.0 = 37.5°C [1] 2 (d) (i) Heat energy released = 25g × 37.5°C × 4.2 = 3937.5 J ≈ 3940 J 1 (ii) Amount of CuSO4 = 0.8 × 25/1000 = 0.02 mol 1 (iii) Enthalpy change = 3.9375kJ ÷ 0.02 mol = –196.875 kJ/mol = –197 kJ/mol 1 (e) Any 1: • Use a lid over the Styrofoam cup to minimize heat loss to the surroundings. • Use a burette/pipette to measure the volume of T (accept: 25 cm3 of T) more accurately. • Use a data logger (reject: machine) with a temperature probe to measure the temperatures more accurately. • Use a magnetic stirrer (reject: electronic stirrer, stirring machine) to ensure more consistent stirring (even distribution of heat energy). 1 20 30 40 50 60 70 80 0 2 4 6