NYJC 2023 H2 Chemistry 9729 P4

[Turn Over NANYANG JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME TUTOR’S NAME CLASS CHEMISTRY 9729/04 Paper 4 Practical 16 August 2023 2 hours 30 minutes Candidates answer on the Question Paper. Additional Materials: As listed in the Confidential Instructions READ THESE INSTRUCTIONS FIRST Write your name and class on all the work you hand in. Give details of the practical shift and laboratory where appropriate, in the boxes provided. Write in dark blue or black pen. You may use an HB pencil for any diagrams, 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. Shift Laboratory For Examiner’s Use 1 / 14 2 / 18 3 / 10 4 / 13 Total / 55 This document consists of 20 printed pages

2 H2 Chemistry 9729/04 NYJC J2/2023 PRELIM 1 Investigation of the reaction between phosphoric acid and sodium hydroxide FA 1 is an aqueous solution of phosphoric acid, H3PO4. FA 2 is 0.115 mol dm–3 sodium hydroxide, NaOH. In this experiment you are to investigate the reaction between phosphoric acid and sodium hydroxide in order to determine the chemical equation. Solution I is thymolphthalein indicator. (a) (i) Procedure Dilution of FA 1 1. Use the pipette to transfer 10.0 cm 3 of FA 1 into a 250 cm3 volumetric flask. 2. Make the solution up to the mark with deionised water and shake well to mix. 3. Label this solution FA 3. Titration of FA 3 against FA 2 4. Fill the burette with FA 2. 5. Use the pipette to transfer 25.0 cm 3 of FA 3 into a 250 cm3 conical flask. 6. Add a few drops of solution I to the conical flask. 7. Run FA 2 from the burette into the conical flask. The end-point is reached when the solution changes from colourless to a permanent pale blue colour. 8. Record your titration results, to an appropriate level of precision, in the space provided below. 9. Repeat points 5 to 8 until consistent results are obtained. Results [4]

3 H2 Chemistry 9729/04 NYJC J2/2023 PRELIM [Turn Over (ii) From your titrations, obtain a suitable volume of FA 2 to be used in your calculations. Show clearly how you obtained this volume. volume of FA 2 = cm3 [1] (b) (i) Calculate the amount of sodium hydroxide present in the volume of FA 2 calculated in (a)(ii). amount of NaOH = mol [1] (ii) The density of the aqueous solution of phosphoric acid, FA 1, is 1.75 g cm–3 and FA 1 contained 8.40 % by mass of H3PO4. Show that the concentration of H3PO4 in FA 1 is 1.50 mol dm–3. [Ar: H, 1.0; O, 16.0; P, 31.0] [2] (iii) Hence, calculate the amount of H3PO4 present in 25.0 cm3 of FA 3. amount of H3PO4 = mol [2]

4 H2 Chemistry 9729/04 NYJC J2/2023 PRELIM (iv) When NaOH reacts with H 3PO4, the salt formed could be NaH 2PO4, Na 2HPO4 or Na3PO4. Use your answer to (b)(i) and (b)(iii) to deduce which one of these three salts was the major product formed during the titration. Write the equation for the reaction of NaOH with H3PO4 to produce this salt. [2] (c) The individual error in any burette reading is ±0.05 cm3. Two students, A and B, record identical burette readings. final burette reading 25.60 cm3 initial burette reading 1.35 cm3 volume added 24.25 cm3 By considering the individual error in each burette reading, explain the following.  The initial burette reading made by student A was 0.05 cm3 greater than the true value but the volume added was exactly 24.25 cm3.  The initial burette reading made by student B was 0.05 cm 3 less than the true value and the actual volume added was exactly 24.15 cm3. [2] [Total: 14]

5 H2 Chemistry 9729/04 NYJC J2/2023 PRELIM [Turn Over 2 Determination of the enthalpy change of solution for ammonium chloride FA 4 is solid ammonium chloride, NH4Cl(s). When an exothermic reaction takes place in a container such as a beaker, some of the evolved heat energy is absorbed by the beaker. When an endothermic reaction takes place some of the required heat energ y is supplied by the beaker. The amount of heat energy evolved or supplied for a 1 °C change in temperature is known as the heat capacity of the beaker. In preparation for your experiment to determine the enthalpy change of solution for FA 4 , you will first need to determine the heat capacity of the 250 cm3 beaker labelled A. When samples of hot and cold water are mixed in beaker A, some heat is lost to the beaker in raising its temperature. To determine the approximate heat capacity of beake r A, you will determine the maximum temperature rise when a sample of hot water is added to cold water in the beaker. (a) (i) Determination of the heat capacity of beaker A 1. Use a 50 cm3 measuring cylinder to transfer 50 cm3 of cold water into beaker A. 2. Use the 50 cm3 measuring cylinder to transfer 50 cm3 of cold water into the other 250 cm3 beaker. Note the temperature of the water in this 250 cm 3 beaker and heat it carefully and gently until the temperature of the water in it has increased by 45 –50 °C then stop heating, e.g. if the water is at 30.0 °C you should warm it to 75–80 °C. 3. Stir the cold water in beaker A with the thermometer. 4. Record the temperature of the cold water (this is the temperature at t = 0.0 min). 5. Record the temperature each minute for 3 minutes. 6. After you have taken the reading at t = 3.0 min, use the thermometer to stir the hot water in the 250 cm3 beaker. 7. At t = 4.0 min, measure the temperature of the hot water and record this value in the box below. The temperature, T1, of the hot water at t = 4.0 min is °C. 8. Immediately add the hot water from the 250 cm3 beaker to the cold water in beaker A. Stir with the thermometer but do not record the temperature.

6 H2 Chemistry 9729/04 NYJC J2/2023 PRELIM 9. Continue to stir the water in beaker A throughout the experiment. 10. Record the temperature at t = 5.0 min, and then every 0.5 minute until t = 8.0 min. 11. Empty and rinse beaker A. Dry it using a paper towel. 12. Record all measurements of time and temperature obtained. Do not include the temperature, T1, of the hot water at t = 4.0 min measured in point 7. Results [3] (ii) Plot a graph of the temperature of the water in beaker A, on the y-axis, against time, on the x-axis, on the grid in Fig. 2.1. Draw a best-fit straight line taking into account all of the points before t = 4.0 min. Draw another best-fit straight line taking into acc ount all of the points after the temperature of the mixture has started to fall steadily. Extrapolate (extend) both lines to t = 4.0 min. Do not plot the temperature, T1, of the hot water at t = 4.0 min.

7 H2 Chemistry 9729/04 NYJC J2/2023 PRELIM [Turn Over Fig. 2.1 [4] (iii) From the extrapolated lines, read the minimum and the maximum temperatures at t = 4.0 min and determine the values for the two temperature changes. Minimum temperature, T2, at t = 4.0 min is