NJC 2025 H2 Chemistry Prelim P3 QP

[Turn over NATIONAL JUNIOR COLLEGE SH2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 3 Free Response Candidates answer on Question Paper. Additional Materials: Data Booklet 9729/03 23 September 2025 2 hours READ THE INSTRUCTIONS FIRST Write your subject class, registration number and name on all the work you hand in. Write in dark blue or black pen. You may use a soft pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Section A Answer all questions. Section B Answer one question. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use Section A 1 /19 2 /20 3 /21 Section B (*circle the question you attempted) 4 /20 5 /20 Paper 3 Total /80 This document consists of 28 printed pages.

2 NJC/H2 Chem Prelim/03/2025 Section A Answer all the questions in this section. 1 (a) A student investigated the thermal decomposition of Group 2 compounds. He heated a mixture of 2.50 g of magnesium nitrate and magnesium carbonate using the setup shown in Fig. 1.1, till no further change was observed. A colourless gas of a volume of 80.8 c m3 was collected at 30 C and atmospheric pressure. Fig 1.1 (i) Write balanced equations for the thermal decomposition of MgCO3 and Mg(NO3)2. [1] (ii) Explain the purpose of sodium hydroxide in the above setup and hence identify the gas collected in the gas syringe. [2] (iii) Calculate the percentage by mass of magnesium nitrate present in the mixture. [3] ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………...

3 NJC/H2 Chem Prelim/03/2025 [Turn over ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... (b) The student carried out another experiment by heating equal amounts of carbonates of magnesium, calcium and barium for two minutes using a Bunsen burner. Table 1.1 shows the volume of gas collected. Table 1.1 Group 2 carbonate MgCO3 CaCO3 BaCO3 Volume of gas collected / cm3 80 25 5 Using relevant data from the Data Booklet, explain the results obtained by the student. [3] ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ……………………………………………………………………………………………………….

4 NJC/H2 Chem Prelim/03/2025 (c) Table 1.2 shows the bond length of various nitrogen-oxygen bonds. Table 1.2 Bond N O N=O nitrogen-oxygen bond in NO3 Bond Length (nm) 0.136 0.115 0.128 Suggest an explanation for the observed NO bond length in nitrate ion. [2] ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. (d) Sodium carbonate, a Group 1 carbonate, can be used to maintain the pH of swimming pool water to the ideal range of 7.0−7.6. If the concentration of sodium carbonate is too high, it will cause skin irritation to swimmers. The management committee of a public swimming pool hired a chemist to advise them on the need to adjust the pH of pool water. The chemist titrated a 10.0 cm 3 sample of pool water (assume it contains only Na2CO3) against 0.05 mol dm−3 HCl. 20.00 cm3 of the HCl solution was required to turn the phenolphthalein indicator from pink to colourless. When the titration is repeated using methyl orange indicator, 40.00 cm 3 of HCl was required to reach the end point. [The Kb1 and Kb2 values of Na2CO3 at 25 C are 2.13 × 10−4 and 2.25 × 10−8 respectively.] (i) The pH at the first equivalence point is found to be greater than 7. Write an equation to explain this observed pH. [1] (ii) Calculate the concentration of Na2CO3 in the pool water sample. [1] (iii) Using your answer to (ii), calculate the initial pH of the pool water sample. [2] (iv) Sketch the pH volume added curve you would expect to obtain when 50.00 cm 3 of the HCl solution is added to 10.0 cm 3 of the pool water sample. Label the various key points on the curve. [2] ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………...

5 NJC/H2 Chem Prelim/03/2025 [Turn over ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………...

6 NJC/H2 Chem Prelim/03/2025 ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... ………………………………………………………………………………………………... (e) Lithium carbonate is a sparingly soluble salt with a Ksp value of 8.15 × 10 4. Calculate the solubility of lithium carbonate. [2] ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………