NJC_2025_H2_Chemistry_Prelim_P3_Ans

[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] MgCO3  MgO + CO2 Mg(NO3)2  MgO + 2 NO2 + ½ O2 (ii) Explain the purpose of sodium hydroxide in the above setup and hence identify the gas collected in the gas syringe. [2] NaOH(aq) reacts with the acidic gases (CO2 and NO2). The gas collected in the gas syringe is O2. (iii) Calculate the percentage by mass of magnesium nitrate present in the mixture. [3] Mg(NO3)2  MgO + 2 NO2 + ½ O2 pV = nRT Amount of O2 gas collected = 101325 × 80.8 × 10−6 8.31×(30+273) = 0.003252 mol Amount of Mg(NO3)2 = 2  0.003252 = 0.006504 mol Mass of Mg(NO3)2 = 0.006504  (24.3+142+166) = 0.9644 g % by mass of Mg(NO3)2 in mixture = 0.9644 2.50  100% = 38.6 %

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] The ionic radius of Mg2+, Ca2+ and Ba2+ are 0.065 nm, 0.099 nm and 0.135 nm respectively. Down Group 2, the ionic charge remains the same while the ionic radius increa