CJC Prelim P4 SOL

1 CANDIDATE NAME CLASS 2T CHEMISTRY 9729/04 Paper 4 Practical 16 August 2017 2 hour 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 in the boxes above. 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 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 15 and 16. 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 15 printed pages and 1 blank page. [Turn over Instruction Catholic Junior College JC2 Preliminary Examinations Higher 2 Shift Laboratory For Examiner’s Use 1 2 3 Total Suggested Solutions

2 Answer all the questions in the spaces provided. 1 'Washing soda' is made from crystals of sodium carbonate, which contains 62.94 % water and 37.06 % sodium carbonate. When stored, these crystals lose some of the water in the crystals to the atmosphere. FA 1 is solid 'washing soda', originally sodium carbonate decahydrate, Na2CO3.10H2O. FA 2 is 0.100 mol dm–3 hydrochloric acid, HCl. In this question, you will perform a titration to determine the amount of water in FA 1 that has been lost to the atmosphere. (a) Method (i) Preparation of FA 3  Weigh the 100 cm3 beaker provided. Record the mass in the space below.  Tip the 'washing soda' crystals, FA 1, into the beaker. Weigh the beaker with FA 1 and record the mass.  Calculate the mass of FA 1 used and record this in the space below.  Add distilled water to the beaker to dissolve the crystals. Carefully transfer the solution to a 250 cm 3 graduated (volumetric) flask labelled FA 3. Wash the beaker twice with small quantities of water and add these washings to the volumetric flask.  Make the solution up to the mark using distilled water. Stopper the flask and invert a number of times to ensure thorough mixing. [1] (ii) Titration of FA 3 against FA 2  Fill the burette with FA 2.  Pipette 25.0 cm 3 of FA 3 into a conical flask and add a few drops of the indicator provided.  Titrate the solution with FA 2. Record your results in the space provided on page 3. Make certain that the recorded results show the precision of your practical work.  Repeat the titration until consistent results are obtained. mass of beaker + FA1 / g 21.138 mass of beaker / g 17.746 mass of FA 1 used / g 3.392

3 Titration Results: [6] (iii) 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 = ……………............… [1] (b) (i) Calculate the amount of hydrochloric acid run from the burette. amount of HCl =………….………....… [1] (ii) Sodium carbonate reacts with hydrochloric acid as follows. Na2CO3(aq) + 2HCl(aq)  2NaCl(aq) + CO2(g) + H2O(l) Calculate the amount of sodium carbonate, Na2CO3, in 250 cm3 of FA 3. amount of Na2CO3 in 250 cm3 of FA 3 =………….………....… [2] mol of Na2CO3 in 25.0 cm3 of FA3 = 2 1 × mol of HCl = 2 1 × 2.55 x 10–3 = 1.275 x 10–3 mol mol of Na2CO3 in 250 cm3 of FA 3 = 0.25 250 × 1.275 x 10–3 = 1.275 x 10 –2 mol 1.275 x 10–2 mol average titre = 𝟐𝟓.𝟓𝟎+𝟐𝟓.𝟓𝟎 𝟐 = 25.50 cm3 mol of HCl = cV = 0.100 × 1000 50.25 = 2.55 x 10–3 mol 2.55 x 10–3 mol 1 2 final burette reading / cm3 28.50 30.50 initial burette reading / cm3 3.00 5.00 volume of FA 2 used / cm3 25.50 25.50   25.50 cm3

4 (iii) Calculate the mass of sodium carbonate, Na2CO3, dissolved in 250 cm3 of FA 3. [Ar: C, 12.0; O, 16.0; Na, 23.0] mass of Na2CO3 in 250 cm3 of FA 3 =………….………....… [1] (iv) Calculate the mass of water present in the washing soda crystals. mass of water =………….………....… [1] (v) Hence calculate the percentage of water in the sodium carbonate crystals, FA 1. percentage of water in FA 1 =………….………....… [1] (c) The maximum error for a 25 cm3 pipette commonly used in schools is 0.06 cm3. The maximum error in any single burette reading is 0.05 cm3. Calculate the maximum percentage error in each of the following. (i) The volume of FA 3 pipetted into the conical flask. maximum percentage error in pipette volume =………….………....… % (ii) The titre volume calculated in (a)(iii). maximum percentage error in titre volume =………….………....… % [2] [Total: 16] Mr of Na2CO3 = 2(23.0) + 12.0 + 3(16.0) = 106.0 mass of Na2CO3 in 250 cm3 FA 3 = nMr = (1.275 x 10–2) x 106.0 = 1.351 g 1.351 g mass of water = mass of FA 1 – mass of Na2CO3 in 250 cm3 of FA 3 = 3.392 – 1.351 = 2.041 g 2.041 g % of water = 392.3 041.2 x 100 = 60.2 % 60.2 % max % error = 0.25 06.0 × 100 = 0.240 % 0.240 max % error = 25.50 )05.02(  × 100 = 0.392 % 0.392

5 2 The enthalpy change for the reaction of sodium carbonate, Na2CO3, with water and carbon dioxide to form sodium hydrogencarbonate, NaHCO 3 cannot be determined directly . However both Na2CO3(s) and NaHCO3(s) react with dilute hydrochloric acid. Reaction 1 NaHCO3(s) + HCl(aq)  NaCl (aq) + H2O(l) + CO2(g) H1 Reaction 2 Na2CO3(s) + 2HCl(aq)  2NaCl (aq) + H2O(l) + CO2(g) H2 In this experiment you will determine the enthalpy change H1 for reaction 1 and H2 for reaction 2, and then use your results to calculate H3 for the reaction: Na2CO3(s) + H2O(l) + CO2(g)  2NaHCO3(s) H3 FA 4 is sodium hydrogen carbonate, NaHCO3. FA 5 is sodium carbonate, Na2CO3. FA 6 is 2.0 mol dm–3 hydrochloric acid, HCl. (a) Method (i) Experiment 1: NaHCO 3(s) + HCl(aq)  NaCl (aq) + H2O(l) + CO2(g)  Use a measuring cylinder to transfer 25 cm3 of the acid, FA 6, into the plastic cup supported in a 250 cm3 beaker. The acid is in excess.  Weigh the container with FA 4 and record the balance reading.  Place the thermometer in the acid and record its temperature at 1 minute interval for the first two minutes.  At 2½ minutes, carefully tip all the FA 4, in small portions, into the acid and stir to dissolve.  Record the temperature of the solution at 1 minute interval from 3 minutes up to 8 minutes.  Reweigh the container with any residual FA 4 and record the balance reading and the mass of FA 4 used.  Rinse out the plastic cup and shake it to remove excess water. Results Record all weighings and temperature readings in the space below. [5] mass of container + FA 4 / g 5.976 mass of container + residual FA 4 / g 3.489 mass of FA 4 used / g 2.487 time / min 0 1 2 3 4 5 6 7 8 temperature / °C 29.6 29.4 29.2 22.2 22.2 22.4 22.8 23.0 23.4

6 (ii) Plot on the grid below, a graph of temperature on the y-axis against time on the x-axis. The scale for the temperature should extend at least 1°C below the lowest recorded temperature. Draw two straight lines of best fit and extrapolate the two lines to 2½ minutes. Determine the change in temperature at this time. temperature change at 2½ minutes = ....................... °C [4] 7.15 temperature change = 29.10 – 21.95 = 7.15 °C

7 (iii) Experiment 2: Na 2CO3(s) + 2HCl(aq)  2NaCl (aq) + H2O(l) + CO2(g)  Use a measuring cylinder to transfer 25 cm3 of the acid, FA 6, into the plastic cup supported in a 250 cm3 beake