VS 2021 O Level Pure Chemistry P3 Answer Scheme

2020 Answer 1 Victoria School 2021 Chemistry O Levels / 6092 Suggested Answer Key PAPER 3 X is ammonium ferrous sulfate. QA video https://www.youtube.com/watch?v=hUOpmteRwOs Question Answer Mark 1 a Remove the stopper from the sample of X in a test-tube. Heat the sample in the test-tube gently for about 1 minute at first and then strongly for about 1 minute. Record all your observations including testing the gas produced with damp red litmus paper. • On gentle heating, the light green solid turned white. [reject blue solid for X] • Condensation occurred. • On strong heating, the white solid turn yellow. • Colourless pungent gas produced turned damp red litmus paper blue. 2pts -1m 2 bi (a) Use a measuring cylinder to transfer about 30 cm3 of deionised water to the 100 cm3 beaker. Add the sample of X from the capped container to the beaker. Stir to ensure all the solid has dissolved, producing solution X. Use solution X to carry out the following tests. You are also provided with Y, hydrogen peroxide. You should test and identify any gases evolved. Record all your observations in the table. The volumes given below are approximate and should be estimated rather than measured unless instructed otherwise. test observation 5

2020 Answer 2 Test 1 Put about 1 cm depth of solution X in a clean test-tube. Add 1cm depth of Y (hydrogen peroxide). Shake the test-tube. Keep this solution for Test 4. • Colourless solution turns red-brown. [1m] • Effervescence of colourless gas was observed. Gas relighted a glowing splint. Oxygen was produced. [1m] • Test-tube feels warm. Test 2 Put about 1 cm depth of solution X in a clean test-tube. Add a few drops of aqueous sodium hydroxide with shaking, then add excess aqueous sodium hydroxide. • Green ppt formed, ppt was insoluble in excess aqueous sodium hydroxide . [1m for here & test 2] Test 3 Put about 1 cm depth of solution X in a clean boiling tube. Add an equal depth of aqueous sodium hydroxide and warm the mixture gently. • green ppt was formed. [1m for here & test 2] • Upon warming, a colourless pungent gas was produced which turned most red litmus paper blue. Ammonia gas was produced. [1m] • Upon standing, green ppt turns red-brown. Test 4 To the mixture from Test 1 add a few drops of aqueous sodium hydroxide with shaking, then add excess aqueous sodium hydroxide. • Red-brown ppt formed, ppt was insoluble in excess aqueous sodium hydroxide. . [1m for test 4 and 5] Test 5 Put about 1 cm depth of solution X in a clean test-tube. Add an equal depth of dilute nitric acid and then add a few drops of aqueous barium nitrate. No reaction was observed when dilute nitric acid was added. • White ppt was formed when aqueous barium nitrate was added. [1m for test 4 and 5] bii Cation: NH4+ and Fe2+ [1m] Anion: SO42- [1m] 2

2020 Answer 3 biii Redox reaction; Hydrogen peroxide was reduced to form water and iron(II) ions were oxidised to iron(III) ions which turns from colourless to brown 1 1 2 ai Titration number 1 2 Final burette reading / cm3 22.30 22.30 Initial burette reading / cm3 0.00 0.00 Volume of P / cm3 22.30 22.30 Best titration results ✓ ✓ 1m: correct headings & correct subtraction 1m: readings to nearest 0.05 cm3 1m: readings used are within ±0.20 cm3 2m: readings are within ±0.20 cm3 of supervisor’s readings 5 aii Average volume of P = (22.30 + 22.30) ÷ 2 = 22.30 cm3 (2 d.p) 1 bi Number of moles of thiosulfate ions = 0.120 x (22.30/1000) = 0.00268 mol 1 bii Number of moles of iodine = 0.00268 ÷ 2 = 0.00134 mol Number of moles of Cu2+ = 0.00134 x 2 = 0.00268 mol Number of moles of Cu2+ in 1 dm3 = 0.00268 ÷ 25/1000 = 0.107 mol 2 biii Mass of copper in brass screw = 0.107 x 64 = 6.85 g 1 biv Percentage by mass of copper = 6.85 ÷ 9.50 x 100% = 72.1% [1m] 1m for units for biii & biv 1m for 3 s.f. 3 c Potassium iodide becomes the limiting reactant, resulting in a smaller number of moles of Cu2+ ions being involved in the reaction, leading to a smaller mass of Cu calculated. Hence, the calculated value for percentage by mass of copper will be lower than expected. 1 1 Titration video: https://www.youtube.com/watch?v=jgBix7HHUYs

2020 Answer 4 3 a Concentration of acid in mol/dm3 = 36.5 ÷ 36.5 = 1.00 mol/dm3 Measure 10 cm3 of hydrochloric acid using a 10 cm3 measuring cylinder and add it into a 100 cm3 beaker. Measure 90 cm3 of water using a 100 cm3 measuring cylinder and add it to the beaker. Using a glass rod, stir and mix the 100 cm3 of 0.100 mol/dm3 hydrochloric acid well. 1 1 1

2020 Answer 5 bi 4 bii Maximum temperature rise = 26.5 – 22.0 = 4.5 °C 1 biii Amount of heat released = 50 x 4.5 x 4.2 = 945 J 1

2020 Answer 6 biv Mass of calcium carbonate = 24.8 – 21.3 = 3.50 g Number of moles of calcium carbonate = 3.50 ÷ 100 = 0.0350 mol 1 bv Amount of heat released = 0.945 ÷ 0.0350 = 27.0 kJ/mol 1 bvi The extrapolation of the graph would allow the temperature rise to be more accurately calculated as the highest temperature measured may not be accurate due to heat loss to the surroundings. 1 c Add excess nitric acid to the sample. Effervescence is observed and the white solid is completely dissolved. Add aqueous ammonia to the solution. If a white ppt is formed, which dissolves in excess, zinc cations are present. 1 1