HCI Prelim Paper 4 Question Paper (final)

HWA CHONG INSTITUTION C2 Preliminary Examination Higher 2 NAME CT GROUP 19S CHEMISTRY Paper 4 Practical Candidates answer on the Question Paper 9729/04 25 August 2020 2 hours 30 minutes 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 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 18 and 19. 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.

2 © Hwa Chong Institution 2020 9729/04/C2Prelim 2020 [Turn over 1 Determination of the percentage by mass of ethanedioic acid in a solid mixture FA 1 is a solution made by dissolving a solid mixture of ethanedioic acid, H2C2O4, and sodium ethanedioate, Na2C2O4, in water. The percentage by mass of ethanedioic acid in the mixture can be determined by a combination of an acid -base titration ( Titration A) and a redox titration (Titration B). You will carry out Titration A only. FA 2 is 0.0400 mol dm–3 sodium hydroxide. You are also provided with thymol blue indicator. Titration A Ethanedioic acid reacts with sodium hydroxide as follows: H2C2O4(aq) + 2NaOH(aq)  Na2C2O4(aq) + 2H2O(l) (a) 1. Fill a burette with FA 2. 2. Pipette 25.0 cm3 of FA 1 into a conical flask. 3. Add about 10 drops of thymol blue indicator. 4. Add FA 2 from the burette until the end-point has been reached. 5. Carry out as many titrations as you think necessary to obtain consistent results. Record your titration results in a suitable format in the space below. [2] (b) From your titration results, obtain a suitable value for the volume of FA 2 needed to react with the ethanedioic acid in 25.0 cm3 of FA 1. volume of FA 2 needed = ………………………… cm3 [3]

3 © Hwa Chong Institution 2020 9729/04/C2Prelim 2020 [Turn over (c) (i) Calculate the number of moles of sodium hydroxide present in the volume of FA 2 found in (b). number of moles of sodium hydroxide = ………………………… mol [1] (ii) Calculate the number of moles of ethanedioic acid present in 25.0 cm3 of FA 1. number of moles of ethanedioic acid = ………………………… mol [1] Titration B In aqueous solution both ethanedioic acid and sodium ethanedioate releas e all their ethanedioate ions, C2O42−. These ions react with manganate(VII) ions in acidic medium. (d) Balance the equation for the reaction between ethanedioate ions and manganate(VII) ions in acidic medium. ….. MnO4−(aq) + ….. H+(aq) + 5 C2O42−(aq) ….. Mn2+(aq) + ….. CO2(g) + ….. H2O(l) [1] 25.0 cm3 of FA 1 was pipetted into a conical flask. Using a measuring cylinder, 30 .0 cm3 of 1.00 mol dm –3 sulfuric acid (an excess) was added. After warming to 70 C, the aliquot was titrated with potassium manganate(VII). 24.80 cm3 of 0.0200 mol dm–3 KMnO4 was required for complete reaction. (e) (i) Calculate the number of moles of manganate(VII) ions required for complete reaction. number of moles of manganate(VII) ions = ………………………… mol [1] (ii) Calculate the total number of moles of ethanedioate ions present in 25.0 cm 3 of FA 1. total number of moles of ethanedioate ions = ………………………… mol [1]

4 © Hwa Chong Institution 2020 9729/04/C2Prelim 2020 [Turn over (f) Use your answers to (c)(ii) and (e)(ii) to calculate the number of moles of sodium ethanedioate, Na2C2O4, present in 25.0 cm3 of FA 1. number of moles of sodium ethanedioate = ………………………… mol [1] (g) Use your answers to (c)(ii) and (f) to calculate the percentage by mass of ethanedioic acid in the solid mixture used to prepare FA 1. [Ar of H: 1.0; C: 12.0; O: 16.0; Na: 23.0] percentage by mass of ethanedioic acid = ………………………… % [2] [Total: 13]

5 © Hwa Chong Institution 2020 9729/04/C2Prelim 2020 [Turn over 2 Determination of the identities of three salts You are provided with solutions FA 3 and FA 4, and solid X in a capped bottle. (a) Preparation of FA 5 solution 1. Weigh the capped bottle containing solid X. 2. Transfer all the solid into a 250 cm 3 beaker. Add about 100 cm 3 of deionised water. Using a glass rod, stir the solution in the beaker until all the solid dissolves. 3. Transfer the solution to the volumetric flask. Rinse the beaker with deionised water several times, adding each rinsing to the flask. 4. Make up the contents of the flask to the 250 cm 3 mark with deionised water. Stopper and mix thoroughly by inverting the flask a number of times. This solution is FA 5. 5. Reweigh the empty bottle and its cap. In an appropriate format in the space below, prepare a table to record all weighings to an appropriate level of precision. Note: You will use FA 5 in Question 2 and Question 3. [1]

6 © Hwa Chong Institution 2020 9729/04/C2Prelim 2020 [Turn over FA 3, FA 4 and the FA 5 you prepared are salt solutions. Each contains one cation and one anion, all of which are different.  FA 3 or FA 4 contains a halide ion.  FA 5 contains Na+ cation and a sulfur-containing anion.  The other five ions are listed in the Qualitative Analysis Notes on pages 18-19. (b) You will identify the cations present in FA 3 and FA 4. To do this, you will carry out four separate tests. You will use aqueous sodium hydroxide, NaOH, and aqueous ammonia, NH3, separately with FA 3 and FA 4. Use about 1 cm depth of each salt solution in a test-tube for each test. If no precipitate forms with aqueous NaOH, transfer the mixture into a boiling tube and carefully warm the boiling tube and its contents. Do not heat until the mixture boils. Note: Keep the product mixture P, obtained from FA 4 and aqueous NH3, for use in part (f). Record all of your observations in a table in the space below. [3] (c) From the bench reagents provided, choose the reagents you would use to identify the halide ion present in FA 3 or FA 4. Test FA 3 and FA 4 with these reagents and record your observations. reagents used ……………………………………………………………………………………… unknown observations FA 3 FA 4 [2]

7 © Hwa Chong Institution 2020 9729/04/C2Prelim 2020 [Turn over (d) Place about 1 cm depth of dilute sulfuric acid, H 2SO4, in a test -tube. Add 1 drop of potassium manganate(VII), KMnO4. Add FA 5 dropwise until no further change is seen. Record your observation. unknown observation FA 5 [1] (e) Use the information given and your observations in (b), (c) and (d) to identify four of the ions. Explain your reasoning, giving the identities of any precipitates formed. FA 3 contains ……………………………………………………………………………………… Explanation ………………………………………………………………………………………… ……………………………………………………………………………………………………….. ……………………………………………………………………………………………………….. FA 4 contains ……………………………………………………………………………………… Explanation ………………………………………………………………………………………… ……………………………………………………………………………………………………….. ……………………………………………………………………………………………………….. FA 5 contains ……………………………………………………………………………………… Explanati