YIJC 2025 Prelim P4 MS (for exchange) H2 Chem

1 © YIJC 9729/04/JC2/PE/2025 YIJC 2025 Prelim Paper 4 Mark Scheme Hierarchy to be used in calculating mean titres in question 1(b)(i): • Value of 2 identical titres • Average of titres within 0.05 cm3 • Average of titres within 0.10 cm3 Use the selected titres to determine the mean titre for the end-point. 1. For calculations, the principle of no double penalty (error carried forward) applies. For connecting parts, marking from point of first penalty onwards will be based on correct method only. 2. Examiners should consider the context and scientific use of any keywords when awarding marks. Although keywords may be present, marks should not be awarded if the keywords are used incorrectly. 3. The examiner should not choose between contradictory statements given in the same question part, and credit should not be awarded for any correct statement that is contradicted within the same question part. Wrong science that is irrelevant to that question should be ignored. 4. Although spellings do not have to be correct, spelling of syllabus terms must allow for clear and unambiguous separation from other syllabus terms with which they may be confused. (e.g., ethane/ethene)

2 © YIJC 9729/04/JC2/PE/2025 Question Answer Marks 1(b)(i) M1 (MMO Quality) SAMPLE DATA Initial burette reading / cm3 4.10 2.20 Burette reading at first end-point / cm3 26.10 24.20 Burette reading at second end-point / cm3 37.20 35.20 Volume of FA 3 used to complete reactions 1 and 2 / cm3 22.00 22.00 Volume of FA 3 used to complete reaction 3 / cm3 11.10 11.00 Total volume of FA 3 used for titration / cm3 33.10 33.00 Has at least two uncorrected titres for end point within 0.10 cm3 for total volume of FA 3 required for the titration. A student’s “rough”/”trial” titre value can be considered by the examiner when selecting titre values for the mean titre calculation if the student has “validated” this value either by ticking it or by using it in an expression in (b)(ii). (By doing either of these, the student has declared it to be no longer a “rough”/”trial” value) 1 M2 (PDO Record) All the burette readings, for all accurate titres in the titration table, are recorded to the nearest 0.05 cm3. Treat all titres as ‘accurate’ unless labelled ‘rough’ or first titre is recorded to a lower precision than subsequent titres. [For arithmetic errors in calculations in table, deduct M5.] 1 M3, M4 (MMO Quality, MMO Accuracy) Calculate the student’s mean titre (based on total volume of FA 3 required for the titration) as described on page 1. Award M2 and M3 based on the difference, ∆ titre, between Student’s and Supervisor’s mean titre. 2

3 © YIJC 9729/04/JC2/PE/2025 Give 2 marks if this difference is ≤ 0.30cm3 Give 1 marks if this difference is > 0.30cm3 but ≤ 0.50cm3 Give 0 marks for a difference > 0.50cm3 1(b)(ii) M5 (MMO Decision) • (For total volume of FA 3 used for the titration): Student obtains appropriate “average” to 2 d.p., from any experiments with uncorrected end-point titre values within 0.10 cm3. • (For volume of FA 3 used to complete reactions 1 and 2 ): Student obtains appropriate “average” to 2 d.p., from the same set of experiments selected in first bullet point above. Do not award this mark if the titres used are not identified either in the table (by, for example, a tick) or in a calculation. Do not award this mark if there are arithmetic errors in the table. 1 1(c)(i) M6 (ACE Interpret) [HCl] in FA 3 = 20.00/250 x 1.5 = 0.120 mol dm-3 M6 1 1(c)(ii) M7, M8 (ACE Interpret) Let the volume of FA 3 required to reach first end-point be Vfirst end point, and total volume of FA 3 required for the titration be Vsecond end point. Vol of FA 3 required to react with NaHCO3 = Vsecond end point – Vfirst end point = VHCl(NaHCO3) M7 Amount of Na2CO3 in 25.0 cm3 of FA 1 = Amount of NaHCO3 formed = Amount of HCl that reacted with NaHCO3 = 0.120 x V/1000 = nNa2CO3 M8 2

4 © YIJC 9729/04/JC2/PE/2025 1(c)(iii) M9, M10 (ACE Interpret) Amount of HCl to react with both Na2CO3 and NaOH = 0.120 x Vfirst end point /1000 = n(Na2CO3 + NaOH) M9 From (c)(ii), we found nNa2CO3 nNaOH = n(Na2CO3 + NaOH) - nNa2CO3 M10 OR Volume of FA 3 reacted with NaOH = Vfirst end point – VHCl(NaHCO3) = VHCl(NaOH) M9 Amount of NaOH in 25.0 cm3 of FA 1 = 0.120 x V HCl(NaOH)/1000 = nNaOH M10 2

5 © YIJC 9729/04/JC2/PE/2025 1(c)(iv) M11, M12 (ACE Interpret) [Na2CO3] in FA 1 = nNa2CO3 x 1000/25 = c1 M11 [NaOH] in FA 1 = nNaOH x 1000/25 = c2 M12 If student cannot obtain nNa2CO3 or nNaOH, they can use 0.00120 mol and 0.00100 mol in place of nNa2CO3 and nNaOH respectively. 1 1 1(d)(i) M13 (ACE interpret) Thymolphthalein colour change from blue to colourless is preferable because it does not obscure or confuse the detection of the second indicator’s colour change. 1 1(d)(ii) M14 (ACE interpret) Since methyl orange is a weak base, it will also react with HCl. More HCl (higher volume of FA 3) is thus required for end-point to be reached. Simply stating more hydrochloric acid is required without any justification warrants no marks at all. 1 1(e) M15 (ACE interpret) Effect: No effect Explanation: If left uncapped, some NaOH in FA 1 will react with CO2 to form Na2CO3 (n(NaOH) : n(Na2CO3) = 2 : 1). However, n(Na2CO3) : n(HCl) = 1 : 2, so the amount of HCl required does not change even though there is less NaOH and more Na2CO3 in uncapped FA 1. 1

6 © YIJC 9729/04/JC2/PE/2025 1(f) M16 (ACE interpret) Discontinuous method requires two separate samples of the NaOH/Na2CO3 mixture. If the samples are not exactly the same , then the titre volumes obtained from both titrations will result in the calculated composition to be inaccurate. OR Performing two separate titrations increases the overall percentage uncertainty , which in turn increases the error in the calculated composition. 1

7 © YIJC 9729/04/JC2/PE/2025 2(a)(i) M17 (PDO layout) Tabulates weighed masses, t and T. Students should record these data in a table or tables. Given the large amount of data to be recorded, students should be allowed to record t and T data in multiple tables. All tables should contain correct headers and units. Missing “capped” – BOD If units have been penalised here, it will not be penalised for M21. 1 M18 (PDO record) Record all masses to 3 d.p. Record temperatures to 0.1 °C Record time to 1 d.p. (0.5 min) Only based on data students presented on page 8. 1 M19 (MMO Collect) Complete set of t and T readings. T readings required at: T= 0.0, 0.5, 1.0, 1.5, 2.0, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0 min AND Complete set of masses • Capped weighing bottle and FA 4 • Emptied weighing bottle plus cap • Mass of FA 4 used (check for arithmetic error) 1 M20 (MMO Quality) Read the Supervisor’s and the student’s temperature at 2.0 min and at 3.0 min . Calculate the T for Supervisor and student. Calculate mass of FA 4 used by student (do not use uncorrected) and also calculate mass of FA 4 used by Supervisor. Then, calculate (T / mass of FA 4) for both the supervisor and the student to 2 d.p. and calculate the difference in these two values. Give 1 mark if the difference is ≤ 1 1

8 © YIJC 9729/04/JC2/PE/2025 2(a)(ii) M21 (PDO Layout) Axes correct way round + correct labels + units + scale Scales chosen so that plotted points occupy at least half graph grid in both x and y directions. The scale must start at zero on the x-axis