NJC Prelim P4 Question Paper

© NJC 2017 9729/04/Prelim17 [Turn over NATIONAL JUNIOR COLLEGE SH2 Year-End Practical Examination Higher 2 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 4 Practical Candidates answer on the Question paper Additional Materials: As listed in the Confidential Instructions 9729/04 17 August 2017 2 hours 30 minutes READ THESE INSTRUCTIONS FIRST Write your identification number and name. Give details of the practical shift and laboratory where appropriate, in the boxes provided. Write in blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. The use of an approved scientific calculator is expected, where appropriate. You may lose marks if you do not show your working or if you do not use appropriate units. Qualitative Analysis Notes are printed on pages 19 and 20. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 20 printed pages including this cover page. Shift Laboratory For Examiner’s use 1 / 12 2 / 16 3 / 10 4 / 17 Total / 55

2 © NJC 2017 9729/04/Prelim17 Answer all the questions in the spaces provided. 1 Determination of x in the formula of hydrated magnesium carbonate, MgCO3.xH2O. Magnesium carbonate is a white, powdery compound that occurs naturally as anhydrous and several hydrated forms. Due to its non-toxicity, it is widely marketed in common products such as cosmetics and toothpaste. A sample of a hydrated magnesium carbonate mineral is analysed to find out the number of moles of water per mole of magnesium carbonate. You are given the following: FA 1 is MgCO3.xH2O solid. FA 2 is 2.00 mol dm–3 nitric acid, HNO3. FA 4 is 9.10 g dm3 sodium hydroxide, NaOH. Indicator: methyl orange In this question, you will perform a back titration. The data from this titration will be used to determine the Mr of MgCO3.xH2O and hence the value of x. (a) In this titration, you will react MgCO3.xH2O with an accurately measured amount of excess FA 2. The reaction mixture was then used to prepare bulk solution FA 3. The amount of unreacted FA 2 in FA 3 is then determined via titration with FA 4. The end -point is reached when the colour of the solution changes from pink to yellow. (i) Preparation of FA 3 by reacting FA 1 with excess FA 2 Record all the mass measurements in the space provided on page 3. 1. Weigh out accurately about 0.20 g of FA 1 in a clean and dry weighing bottle. 2. Transfer FA 1 to a clean and dry small beaker. Measure and record the mass of weighing bottle and residual solid. Calculate the actual mass of FA 1 transferred. 3. Pipette 25.0 cm3 of FA 2 to dissolve ALL solid FA 1. 4. Transfer the resultant solution to a 250 cm3 volumetric flask. 5. Use a small volume of deionised water to rinse the inner wall of the small beaker and the glass rod. Transfer ALL washings to the volumetric flask. 6. Add more deionised water to make up the volume to the mark. Stopper and shake to ensure complete reaction and to obtain a uniform solution. 7. Label this solution as FA 3.

3 © NJC 2017 9729/04/Prelim17 [Turn over (ii) Titration of FA 3 against FA 4 8. Fill the burette with FA 4. 9. Using a pipette, transfer 25.0 cm3 of FA 3 into a conical flask and titrate the unreacted HNO3 with FA 4, using methyl orange as indicator. 10. Run FA 4 from the burette into this flask until solution changes from pink to yellow. 11. Record your titration results in the spaces provided below. Make certain that your recorded results show the precision of your instrument used. 12. Repeat steps 9 to 11 until your results are within ± 0.10 cm3. Results [5] (iii) From your titrations, obtain a suitable volume of FA 4 to be used in your calculations. Show clearly how you obtained this volume. Volume of FA 4 = …………………… [1]

4 © NJC 2017 9729/04/Prelim17 (b) (i) Using your titration results, calculate the amount of unreacted HNO3 in 25.0 cm3 of FA 3. Hence calculate the total amount of unreacted HNO3 in 250 cm3 of FA 3. [Ar: H, 1.0; O, 16.0; Na, 23.0] Amount of unreacted HNO3 in 250cm3 of FA 3 = ………………. [1] (ii) Determine the total amount of HNO3 in 25.0 cm 3 of FA 2 before reacting with MgCO3.xH2O. Amount of HNO3 in 25.0 cm3 of FA 2 = ………………... (iii) Hence calculate the amount of HNO3 used to react with MgCO3.xH2O. Amount of HNO3 reacted with MgCO3.xH2O = ………………. [1]

5 © NJC 2017 9729/04/Prelim17 [Turn over (iv) Determine the relative molecular mass of MgCO3.xH2O. Mr of MgCO3.xH2O = ……………………. Hence calculate the value of x in MgCO3.xH2O. Give your answer to the nearest whole number. [Ar: H, 1.0; C, 12.0; O, 16.0; Mg, 24.3] x is ………..……….[2] (c) Another student repeated the experiment using the procedure in (a), however in step 6, he made up the solution to the 250 cm 3 mark by adding FA 2 instead of deionised water. Explain the effect on his titre value and why it is unsuitable to do so. …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………[2] [Total:12]

6 © NJC 2017 9729/04/Prelim17 2 Determination of the enthalpy change for the formation of hydrated magnesium carbonate, Hreaction1. In this experiment, you are to determine Hreaction3. Hreaction1 will be determined using Hess’s law. Mg(s) + C(s) + (3+x)/2 O2(g) + xH2(g)  MgCO3.xH2O(s) Hreaction1 Mg(s) + 2HNO3(aq)  Mg(NO3)2(aq) + H2(g) Hreaction 2 = ‒ 490 kJ mol−1 MgCO3.xH2O(s) + 2HNO3(aq)  Mg(NO3)2(aq) + CO2(g) + (x+1)H2O(l) Hreaction3 FA 1 is MgCO3.xH2O solid. FA 2 is 2.00 mol dm3 nitric acid, HNO3. (a) You are to perform the experiment below. Determination of Hreaction3 1. Place a Styrofoam cup with lid inside a second Styrofoam cup which is held in a glass beaker to prevent it from tipping over. 2. Using a measuring cylinder, place 40.0 cm3 of FA 2 into the first Styrofoam cup. Measure and record its initial temperature, Ti. 3. Measure accurately about 1.00 g of FA 1 in a weighing bottle. 4. Transfer FA 1 into the Styrofoam cup containing FA 2 and quickly replace the lid. Stir and measure the highest/lowest temperature obtained. Record this temperature, Tf. 5. Reweigh the weighing bottle. 6. Wash and dry the Styrofoam cups. 7. Record your data in the space provided below. Make certain that the recorded results show the precision of the instrument used. Results [3]

7 © NJC 2017 9729/04/Prelim17 [Turn over (i) Using your results, c alculate the amount of heat change, Δqreaction3, by reaction 3: [c = 4.18 Jg1K1, density of water = 1.00 g cm3] Hreaction3 MgCO3.xH2O(s) + 2HNO3(aq)  Mg(NO3)2(aq) + CO2(g) + (x+1)H2O(l) Δqreaction3 = …………….[1] (ii) Calculate the Hreaction3. (If you were unable to calculate x in 1(b)(iv), you may assume that x = 3 for the calculation here and in part (b). Note: this is a hypothetical value.) Hreaction3 = …………….[1] (b) Determination of Hreaction1 It is given that Hfθ of CO2(g) and H2O(l) is −393.5 kJ mol−1 and −285.8 kJ mol−1 respectively. Using these values toge