SJI 2021 Y4 Prelim Chemistry 6092 P3 Answers

1 St. Joseph’s Institution 2021 Year 4 OP Preliminary Exam Answer Scheme Paper 3 Question 1 Qn Suggested answers Mark 1a & 1b Skills: MMO & PDO Results table: For experiments 1 & 2: Records correct headings and units in the table. For experiment 1: All readings of volume are recorded to nearest 0.5 cm3 All readings of volume show an increasing trend. For experiment 2: All readings of volume are recorded to nearest 0.5 cm3. All readings of volume show an increasing trend. 1 1 1 1 1 1c Skills: PDO Graph A smooth curve is drawn (for both graphs) Points are plotted correctly Graphs are labelled & axes are labelled with units Suitable scale is used 1 1 1 1 1di Experiment 1 [must state but no mark] Experiment 1 has a steeper initial gradient than Experiment 2. (the gradient indicates the rate of reaction) 1 1dii Acid X has double the concentration (or higher concentration) of hydrogen ions compared to acid Y / Acid X is dibasic while acid Y is monobasic. 1 1diii Calculations to be shown clearly i.e. volume at 30 seconds indicated on the graph and answer given to 3 s.f. Units correctly indicated 1 1 1e To remove the oxide layer, enabling the reaction between the metal and acid to occur directly. 1 1f The acids can be run into the conical flask directly from burettes, which measure volume to an accuracy of +/- 0.05 cm3. / Burette should be used to measure volume instead of measuring cylinder 1 1g The carbon dioxide gas formed in the reaction is soluble in water, causing the volume of gas measured to be lower than expected. A gas syringe should be used to collect and measure the volume of carbon dioxide gas instead of using collection over water. 1 1 TOTAL 17

2 Paper 3 Question 2 Qn Suggested answers Mark 2ai White ppt formed 1 2aii Ppt is soluble in excess aqueous NaOH, forming a colourless solution. 1 1 2b White ppt formed which is insoluble in excess aqueous NH3. 1 1 2c White ppt is formed. Ppt is insoluble in dilute nitric acid. 1 1 2d Aluminium sulfate 1 2e Green solid turns black 1 2f Effervescence of colourless gas observed Gas evolved forms a white ppt with limewater Presence of carbon dioxide. 1 1 1 2gi Blue ppt formed. The ppt is insoluble in excess aqueous NaOH. 1 1 2gii Blue ppt formed. The precipitate is soluble in excess aqueous NH3, forming a dark blue solution. 1 1 1 2h copper(II) carbonate 1 TOTAL 18 Paper 3 Question 3 Qn Suggested answers Mark 3 Procedure: 1. Measure the mass of the rusty nail using an electronic balance, M g. 2. Add the rusty nail into a conical flask and add excess dilute acid to the conical flask. 3. Stopper the conical flask immediately with a delivery tube connected to a calibrated gas syringe. 4. Measure the maximum volume of gas collected in the syringe, V cm3. 5. Calculate the mass of rust using the following method: Fe + 2H+ → Fe2+ + H2 No. of moles of hydrogen = (V/24000) mol No. of moles of iron = (V/24000) mol Mass of iron = (V/24000) x 56 = (7V/3000) g Mass of rust = M – (7V/3000) g Assumption: Only iron in the rusty nail reacts with the acid to form hydrogen gas. / The rusty nail does not contain any other substance that reacts with the acid to produce a gas, causing the volume of gas collected to be higher than expected. 5 TOTAL 5