VS 2021 Chemistry 6092 P3 Answer Key

2020 Answer 1 Victoria School 2021 Chemistry O Levels / 6092 Suggested Answer Key PAPER 3 Question Answer Mark 1 a On gentle heating, the light green solid turned white. Colourless liquid was formed on the walls of the test-tube. On strong heating, the white solid turn yellow. Dark brown (to near black) solid was formed. Colourless pungent gas produced turned damp red litmus paper blue. 2 bi test observation Test 1 Colourless solution turns reddish-brown. Effervescence of colourless gas was observed. Gas relighted a glowing splint. Oxygen was produced. Heat was given out. Test 2 Dirty green ppt was formed Ppt was insoluble in excess aqueous sodium hydroxide Test 3 Dirty green ppt was formed. Upon warming, a colourless pungent gas was produced which turned most red litmus paper blue. Ammonia gas was produced. Upon standing, green ppt turns red-brown. Test 4 Reddish-brown ppt was formed. The orange-brown solution turned to light yellow. The ppt was insoluble in excess aqueous sodium hydroxide. Test 5 No reaction was observed when dilute nitric acid was added. White ppt was formed when aqueous barium nitrate was added. 5 bii Cation: NH4+ and Fe2+ 2

2020 Answer 2 Anion: SO42- biii Redox reaction. Hydrogen peroxide was reduced to form water and iron(II) ions were oxidised to iron(III) ions as seen from test 1 where the colourless solution turns brown. 2 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   5 aii Average volume of P = (22.30 + 22.30) ÷ 2 = 22.30 cm3 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% 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. 2 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. 3

2020 Answer 3 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 4 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. 2