EJC Prelim P4

© EJC 9729/04/J2PE/18 [Turn Over EUNOIA JUNIOR COLLEGE JC2 Preliminary Examination 2018 General Certificate of Education Advanced Level Higher 2 CANDIDATE NAME CIVICS GROUP 1 7 – INDEX NUMBER CHEMISTRY Paper 4 Practical 9729/04 28 August 2018 2 hours 30 minutes Candidates answer on the Question Paper. Additional Materials: As listed in the Confidental Instructions READ THESE INSTRUCTIONS FIRST Write your name, civics group and registration number on 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 or graphs. Do not use paper clips, highlighters, 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 2 3 4 Total This document consists of 19 printed pages and 1 blank page.

2 © EJC 9729/04/J2PE/18 For Examiner’s Use Answer all the questions in the spaces provided. 1 Analysis of a solution containing both sodium hydroxide and sodium carbonate by Warder titration (double indicator method) Aqueous sodium hydroxide easily reacts with atmospheric carbon dioxide. This results in the sodium hydroxide being contaminated with sodium carbonate Na2CO3. The Warder titration (double indicator method) can be used to analyse a mixture containing both sodium hydroxide and sodium carbonate. FA 1 is 0.125 mol dm –3 hydrochloric acid, HCl. FA 2 is an aqueous solution containing sodium hydroxide, NaOH, and sodium carbonate, Na2CO3. (a) Titration of FA 2 against FA 1 using methyl orange or screened methyl orange indicator In this titration, FA 1 is run from the burette into the conical flask containing FA 2 with methyl orange or screened methyl orange indicator. The end-point is reached when the yellow colour changes to orange for methyl orange, or the green colour changes to grey for screened methyl orange. (i) 1. Fill the burette with FA 1. 2. Using a pipette, transfer 25.0 cm 3 of FA 2 into a conical flask. 3. Add a few drops of methyl orange or screened methyl orange indicator. 4. Run FA 1 from the burette into the conical flask until the colour of the solution changes. 5. Record your titration results in the space provided below. Make certain that your recorded results show the precision of your working. 6. Repeat points 1 to 5 as necessary until consistent results are obtained. Results [5]

3 © EJC 9729/04/J2PE/18 [Turn Over For Examiner’s Use (ii) From your titrations, obtain a suitable volume of FA 1 to be used in your calculations. Show clearly how you obtained this volume. Volume of FA 1 = ................................................ [1] (b) Titration of FA 2 against FA 1 using thymolphthalein indicator When the titration in (a) was repeated using thymolphthalein as the indicator, 25.0 cm 3 of FA 2 required 23.35 cm3 of FA 1. The following explains why different results were obtained using two different indicators. • When thymolphthalein is used as the indicator, the following reactions would have taken place at the end-point of the titration. 1. NaOH(aq) + HC l(aq) → NaCl(aq) + H 2O(l) 2. Na 2CO3(aq) + HCl(aq) → NaCl(aq) + NaHCO3(aq) • When methyl orange is used as the indicator in (a), the following reactions would have taken place at the end-point of the titration. 1. NaOH(aq) + HC l(aq) → NaCl(aq) + H 2O(l) 2. Na 2CO3(aq) + HCl(aq) → NaCl(aq) + NaHCO3(aq) 3. NaHCO 3(aq) + HCl(aq) → NaCl(aq) + CO2(g) + H2O(l) (i) Calculate the amount of hydrochloric acid present in the volume of FA 1 calculated in (a)(ii). amount of HCl in volume in (a)(ii) = .................................................. [1] (ii) Calculate the amount of hydrochloric acid in 23.35 cm 3 of FA 1. amount of HCl in 23.35 cm 3 of FA 1 = ................................................... [1]

4 © EJC 9729/04/J2PE/18 For Examiner’s Use (iii) Using your answer to (b)(i) and (b)(ii), calculate the amount of hydrochloric acid that reacts with the Na2CO3 in the titration using thymolphthalein indicator. amount of HCl that react with Na 2CO3 = ................................................ [1] (iv) Hence, calculate the mass of sodium carbonate present in 25.0 cm 3 of FA 2. [Ar: C, 12.0; O, 16.0; Na, 23.0] mass of Na 2CO3 in 25.0 cm3 of FA 2 = ................................................ [1] (v) The overall equation for the reaction of Na 2CO3 with HCl when methyl orange is used as indicator is given below. Na2CO3(aq) + 2HCl(aq) → 2NaCl(aq) + CO2(g) + H2O(l) Calculate the amount of HCl that reacted with the Na 2CO3 in the above equation in 25.0 cm3 of FA 2. amount of HCl = ................................................ [1] (vi) Use your answers to (b)(i) and (b)(v) to calculate the mass of sodium hydroxide in 25.0 cm 3 of FA 2. [A r: H, 1.0; O, 16.0; Na, 23.0] mass of NaOH = ............................................... [1]

5 © EJC 9729/04/J2PE/18 [Turn Over For Examiner’s Use (vii) Calculate the percentage by mass of sodium carbonate in the mixture of sodium hydroxide and sodium carbonate in FA 2. percentage by mass of Na 2CO3 in FA 2 = .................................................... [1] (c) The error (uncertainty) associated with each reading is given as follows: 50.00 cm3 burette: ±0.05 cm 3 25.0 cm3 pipette: ±0.04 cm 3 Using the above data, calculate the percentage error (uncertainty) when a (I) 50.00 cm 3 burette (II) 25.0 cm3 pipette is used to measure 25 cm3 of FA 2 into the conical flask. Hence state whether a 50.00 cm 3 burette or 25.0 cm 3 pipette will be more suitable to measure 25.0 cm3 of FA 2 accurately. ....................................................................................................................................... ....................................................................................................................................... ....................................................................................................................................... ....................................................................................................................................... ....................................................................................................................................... ....................................................................................................................................... .................................................................................................................................. [2] [Total: 15]

6 © EJC 9729/04/J2PE/18 For Examiner’s Use 2 To determine the percentage by mass of sodium hydrogencarbonate in the unknown sample based on gravimetric method Sodium hydrogencarbonate decomposes on heating to give sodium carbonate, carbon dioxide and water vapour. () () () ()32 3 2 22NaHCO s Na CO s CO g H O gΔ⎯⎯→+ + Sodium chloride on the other hand is unaffected by heat. FA 3 is a mixture of anhydrous sodium hydrogencarbonate, NaHCO 3, and sodium chloride, NaCl. In this question, you will heat to dec ompose the sodium hydr