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