SJI 2022 Y4 Prelim Chemistry 6092 Paper 3 AS

1 St. Joseph’s Institution 2022 Year 4 OP Preliminary Exam Answer Scheme Paper 3 Question 1 Qn Suggested answers Mark 1a record temperatures to 1 d.p., correct trend correct calculation of temperature rise 1 1 1b record temperature to 1 d.p., correct trend (temperature increases more than Expt 1) correct calculation of temperature rise 1 1 1c all points plotted correctly for both experiments 2 straight lines of best fit which intersects lines passes through origin appropriate scale chosen, graphs are labelled clearly 1 1 1 1 1di correct reading from the graph with units, working shown clearly on graph 1 1dii No. of moles of acid = x / 1000 x 1.00 = y mol No. of moles of alkali = y mol Concentration of sodium hydroxide = y / (25.0 / 1000) = z mol/dm3 1 1 1e exothermic reaction 1 1f The burette is rinsed with acid B to remove traces of water / prevent acid B from being diluted 1 1g Experiment 2 Acid B is a dibasic acid / stronger acid / more acidic / has a lower pH. 1 1h Heat loss to the surroundings. Place a lid over the Styrofoam cup to reduce heat loss. 1 1 TOTAL 16 Paper 3 Question 2 Test no. Test Observations 1 Place one spatula of solid W into a test tube. Add about 2 cm 3 of dilute hydrochloric acid to solid W. Effervescence of colourless gas [1] White ppt formed when gas is bubbled into limewater [1] Gas is carbon dioxide [1] White solid dissolves to form a colourless solution [1] 2 Place one spatula of solid W into a test tube. Add about 5 cm 3 of aqueous copper(II) sulfate and heat the mixture gently. Leave the test tube to stand. Green solid / ppt formed [1] Blue solution turns colourless [1]

2 3 Place one spatula of solid W into a test tube. Add one spatula of ammonium chloride to solid W and shake the test tube. Heat the mixture gently. Colourless droplets of liquid formed along sides of test tube [1] Pungent gas evolved [1] Moist red litmus turns blue [1] Gas is ammonia [1] [10 observations, maximum 8 marks] [8] Qn Suggested answers Mark Markers’ Comments 2a Solid W contains carbonate ion and is basic. 1 1 TOTAL 12 Paper 3 Question 3 Qn Suggested answers Mark 3a all points plotted correctly smooth curves for Experiments 1 and 2 1 1 3bi correct reading from the graph with units accept: 30 – 34 cm3 1 3bii correct working with units accept: 10 – 14 cm3 1 3c copper(II) oxide acts as a catalyst. 1 3d Potassium(V) chlorate has decomposed completely. / All potassium(V) chlorate has decomposed. 1 3e No. of moles of O2 = 72 / 24000 = 0.00300 mol No. of moles of KClO3 = 0.00300 / 3 x 2 = 0.00200 mol Mass of KClO3 = 0.00200 x 122.5 = 0.245 g 1 1 1 3f Suggested answer: ● Measure the initial mass of the solid mixture (m1) using an electronic balance. ● Place the mixture into a conical flask, then add excess acid. ● Immediately stopper the conical flask with a delivery tube connected to a calibrated gas syringe. ● Measure the volume of carbon dioxide gas (V cm3) in the gas syringe after there is no more effervescence / the reaction is complete. ● Calculate the mass of copper(II) carbonate: o No. of moles of CO2 = V / 24000 mol o No. of moles of CuCO3 = V / 24000 mol o Mass of CuCO3, m2 = V / 24000 x 124 g ● The percentage by mass of copper(II) carbonate can be found using: [(m1 – m2) / m1] x 100% ● Assumption: The solid mixture does not contain any other impurities that can react with the acid to produce a gas. 5 TOTAL 14