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 = 1: x = [(d-a)/(106 which is Mr of Na2CO3)] : [(b-d)/(18 which is Mr of H2O)] [1] Apparatus – Boiling tube (or suitable apparatus, e.g. crucible/evaporating dish) AND electronic (mass) balance (REJECT: test-tube/beaker/conical flask…) [1] Recording of “mass of empty boiling tube/g” AND “mass of boiling tube and Na2CO3 crystals (before heating)/g” [1] Recording of “mass of boiling tube and content (after heating)/g” [1] Calculation of Steps 7 & 8 to determine the value of x [6]

3 2ai mass of sodium carbonate added = 5.12 g (acceptable range of 4.80g – 5.25g) [2] time / min 0 1 2 3 4 5 6 7 8 temperature / oC 29.0 29.0 29.0 32.5 32.0 32.0 32.0 32.0 [1] Values of BOTH mass measurements to 2 decimal places and mass of Na2CO3 added is within range [1] Values of ALL temperature measurements to nearest 0.5 oC

4 2aii [5]

5 [1] All plotted points must be present (but allow for one error) [1] Axis labels AND units [1] Appropriate scales of both axes (e.g. in 2/5/10) [1] Best-fit straight lines (i.e. no curve, connect-the-dots, crooked/thick lines and MUST include straight line at 3rd min with connections to 2 straight lines before and after addition) Maximum temperature change = 32.6 - 29.0 = + 3.6 oC [1] (MUST be (i) based on graph drawn, (ii) dotted lines, (iii) 1.d.p and (iv) workings) 2aiii Amount of heat change = (100)(3.6)(4.2) = 1512 J = 1510 J [1] 2aiv No. of moles of Na2CO3 used = 5.12 / 106 = 0.048302 mol = 0.0483 mol [1] 2av Enthalpy change of reaction = - (1512 / 0.048302) = - 32945 J/mol = - 31.3 kJ/mol [1] Numerical answer and sign for enthalpy change of reaction [1] Answers to 3 significant figures for (a)(iii), (a)(iv) AND (a)(v). [2] 2avi error: Acid spray resulting in loss of volume of solution [1] in the Styrofoam cup. OR Heat loss to the surrounding effect on the value of enthalpy change of reaction calculated: Since the volume of solution in the Styrofoam cup is less, the amount of heat change calculated will be less than expected OR the highest temperature recorded is lower than expected AND the value of the enthalpy change of reaction will be smaller (than expected) [1]. improvement: Cover the Styrofoam cup with a (plastic) lid [1] with a hole for the thermometer, immediately after adding sodium carbonate. [3] 2b Add (excess) aqueous sulfuric acid (or H 2SO4 (aq)) [1] to a sample of sodium carbonate. If barium carbonate is present, a white precipitate [1] (of barium sulfate) will be observed. Otherwise, if barium carbonate is not present, there will be no precipitate formed. [2]

6 3a test observations 1 The solid dissolves to form a green solution [1]. 2 A green precipitate is formed, which is insoluble in excess NaOH (aq) [1]. No observable change [1] 3 A white precipitate is formed in a green solution[1]. * Note that any contradictory observations in above table (e.g. mentioning “effervescence”, when no effervescence evolved), student will be awarded [0]. For subsequent questions in (b) and (c), answers cannot contradict the observations seen, otherwise be awarded [0]. [4] 3b Although a green precipitate is formed when NaOH (aq) is added, the green precipitate did not turn reddish-brown when the test-tube is left to stand [1]. This confirms that Fe2+ (aq) is absent. * Note that this mark can only be awarded if student did not write contradictory observation based on Test 2. [1] 3c When HNO3 (aq) is added to the Y, there was no effervescence observed, thus confirming that CO32- (aq) is absent. * Note that this mark can only be awarded if student did not write contradictory observation based on Test 1. [1] End of Paper