Copy of PLMGSS 2024 4E Chem P3 Ans

1 Answer to Sec 4 Prelim Exams 2024 Qn Answer Mark 1(a)(i) Final burette reading /cm3 18.70 28.90 28.90 Initial burette reading /cm3 0.00 10.00 10.00 Volume of P used /cm3 18.70 18.90 18.90 Reading chosen ✓ ✓ A-Accuracy (average titre within 0.20 cm3 of teacher’s average tire value) [2] (average titre within 0.10 cm3) [1] C- Concordance (two accurate titre values are within 0.20cm3) [1] T- [2] ✓ The table headings and units has to be correct ✓ All burette readings for all accurate titres in titration table are recorded to nearest 0.05 cm3 [5] 1(a)(ii) Average volume of P = (18.90 + 18.90) / 2 = 18.90 cm3 [1] 1(b)(i) Number of moles of sodium hydroxide = 0.200 x 18.90/1000 = 0.00378 mol Number of moles of HA = 0.00378 mol [1] 1(b)(ii) In 25 cm3, there are 0.00378 moles of HA. In 1000 cm3, there are (0.00378/25) x 1000 = 0.1512 mol Concentration in cleaning product = 0.1512 / 100/1000 = 1.51 mol.dm3 Working – [1], answer – [1] 1(c)(i) 1) Measure 25.0 cm3 of aqueous sodium carbonate using a measuring cylinder and pour the solution into a conical flask. 2) Weigh 1.0 g of solid HA using an electronic balance and place the solid into a small vial. Carefully lower the solid HA into the conical flask, ensuring the chemicals do not mix. 3) Set up the apparatus as shown above. 4) Pull the string to allow the solid HA to react completely with the aqueous sodium carbonate. 5) Assume all the solid HA has reacted with the aqueous sodium carbonate (in excess). 6) Record the total volume of carbon dioxide gas collected in the measuring cylinder when reaction is completed (no more effervescence or more further change in the volume of CO2) in the conical flask. 7) Calculate the number of moles of CO2 produced using the formula: volume (in cm3) / 24 000 cm3 = x mol 8) Using the mole ratio HA:Na2CO3 = 2:1 Number of moles of HA = 2x mol 9) Using the formula, number of moles of HA = mass of HA / Mr of HA 2x = 1/Mr of HA Mr of HA = 1/2x Apparatus (measuring cylinder, electronic balance) – [1] Measurement: excess aqueous sodium carbonate – [1] Measurement: Total volume of carbon dioxide collected – [1] Calculation for number of moles of HA – [1] Calculation for Mr– [1]

2 1(c)(ii) Since CO2 is soluble in water, the total volume of CO 2 collected would be lower than actual, which will cause the number of moles of CO2 as well as number of moles of HA to be lower than expected. As a result, the Mr of HA will be larger than actual. [1] [1] [1] 1(c)(iii) Change: Collect the volume of CO2 produced by using a gas syringe instead of displacement of water. Explanation: Volume of CO2 will be the true volume as water is absent in the collection of CO2. [1] 2(a)(i) metal carbonate X: carbonate of metal R Y: carbonate of metal S Colour of carbonate before heating white white Colour change during heating/cooling Yellow when hot White