Copy of ACSBR 6092 Sec 4 Prelims 2024_P3_ANS

1 Answers to 2024 Preliminary Examination 6092/3: No. Answers Marks Comments 1ai [1] Records initial (burette) readings, final (burette) readings and volume of P added with correct headings and units in the titration table. [NO MARK if there are repeat units within each reading in table] [1] All burette readings for all titres in titration table are recorded to nearest 0.05 cm3 (2 d.p.). [1] Determine concordance with at least two UNCORRECTED titres within ±0.20 cm3. [2] Determine accuracy of candidate average titre using CORRECTED values… • 2 marks: (±0.20 cm3) within 23.20 – 23.60 cm3 • 1 mark: (±0.30 cm3) within 23.10 – 23.70 cm3 [5] 1aii Average volume of P = (Reading 1 + Reading 2) / 2 = 23.40 cm3 [1] [1] 1aiii No. of moles of P reacted = 1.50 x (23.40/1000) = 0.0351 mol [1] Based on mole ratio, no. of moles of Na2CO3 in Q reacted = 0.0351 / 2 = 0.01755 mol Molar concentration of Q = 0.01755 / (25.0/1000) = 0.702 mol/dm3 [1] (if final answer is not in 3.s.f., deduct one mark) [2] 1aiv Mr of Na2CO3•xH2O = 200 / 0.702 = 284.900 = 283.9 [1] Mr of Na2CO3•xH2O = 106 + x (18) = 284.900 x = 9.94 ≈ 10 (nearest whole number) [1] [2] * Note that appropriate average volume of ___ from closest titre values (titres should be identified either in the table by a tick or in a calculation AND within 0.20 cm3 of each other). REJECT if answer is not in 2 d.p. or rounding error OR ticked values and calculations mismatch from student’s selected titre values.

2 1av After rinsing, some of Q would have reacted with (residual) P (HCl) OR Q is neutralised with P, hence the number of moles of Q transferred to the conical flask would be less (than expected) [1]. The Mr of Na2CO3•xH2O will therefore be larger (than expected), and the (value of x OR) amount of water of crystallisation would also be larger (than expected) [1]. * Note that student must show understanding of “water of crystallisation”, otherwise award [0]. [2] 1b 1. Prepare a recording table as follows: Mass of empty boiling tube / g a Mass of boiling tube and Na2CO3 crystals (before heating) / g b Mass of boiling tube and content (after 1st heating) / g c Mass of boiling tube and content (after 2nd heating) / g d Mass of boiling tube and content (after 3rd heating) / g d 2. Using an electronic mass balance, measure and record the mass of an empty boiling tube. 3. Transfer the Na 2CO3 crystals to the boiling tube. Measure and record the total mass of boiling tube and Na2CO3 crystals. 4. Heat the boiling tube for approximately 2-3 minutes (max. of 5 mins) and leave the boiling tube to cool to room temperature [1]. 5. Reweigh the boiling tube and its content and record the mass (after 1st heating). 6. Repeat Steps 4 and 5 until a constant mass is obtained [1]. 7. Calculate the mass of water of crystallisation lost: mass of water of crystalliation = (b-d) g 8. Calculate the value of x: nNa2CO3 : nH2O