2019 CJC Prelim H2 Chem P4 ANS

CANDIDATE NAME CLASS 2T CHEMISTRY 9729/04 Paper 4 Practical 19 August 2019 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 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 14 and 15. 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 16 2 14 3 13 4 12 Total 55

2 9729/04/CJC JC2 Preliminary Examination 2019 1 Determination of the percentage by mass of iron(II) sulfate in an iron tablet Iron tablets, taken for the prevention and treatment of iron deficiency, are health supplements readily available in pharmac ies. These iron tablets contain iron(II) sulfate, which is a soluble form of iron. Assuming that all the iron in the tablets is in the form of Fe2+, it is possible to estimate the iron content by titration against potassium manganate(VII). The equation for this reaction is 5Fe2+(aq) + MnO4–(aq) + 8H+(aq)  5Fe3+(aq) + Mn2+(aq) + 4H2O(l) You are provided with the following. FA 1 contains 5 iron tablets, crushed to a powder. FA 2 is 0.00500 mol dm–3 potassium manganate(VII), KMnO4. 1.0 mol dm–3 sulfuric acid, H2SO4. deionised water In this experiment, you will determine the percentage of iron(II) sulfate heptahydrate, FeSO4.7H2O, present in the iron tablets FA 1. (a) Preparation of FA 3, a solution of FeSO4.7H2O.  Use a measuring cylinder to transfer about 100 cm3 of 1.0 mol dm–3 sulfuric acid into a conical flask.  Determine accurately the mass of the crushed iron tablets used, FA 1, and record your readings in the space below. mass of the crushed iron tablets used = ……………. g  Add all the crushed tablets, FA 1, to the sulfuric acid.  Warm the mixture gently, do not overheat, and stir for about two minutes.  Allow the flask to cool for around five minutes.  Filter the mixture into a 250 cm3 volumetric flask. Ensure that no solution is lost. Note: Th e filtration takes time . Proceed to Question 2 whilst the mixture is filtering.  Wash out the conical flask with deionised water and add the washings through the filter into the volumetric flask.  Make up the contents of the flask to the 250 cm3 mark with deionised water. Stopper and mix the contents thoroughly to obtain a homogeneous solution.  Label this solution FA 3. mass of container + FA 1 / g m2 mass of container + residual FA 1 / g m1 mass of FA 1 used / g m2 – m1

3 9729/04/CJC JC2 Preliminary Examination 2019 (b) Titration of FA 3 with FA 2  Fill the burette with FA 2.  Pipette 25.0 cm3 of FA 3 into a conical flask.  Use a measuring cylinder to add about 25.0 cm3 of 1.0 mol dm–3 sulfuric acid to the conical flask.  Titrate with FA 2 until the appearance of a first permanent pale-pink colour.  Record your titration results in the space below. Make certain that the recorded results show the precision of your practical work.  Repeat the titration as many times as necessary to obtain reliable results. Titration Results [6] 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] (c) (i) Calculate the amount of MnO4– ions contained in the mean titre. amount of MnO4– ions =………….………....… [1] average titre = 𝑽𝟏+𝑽𝟐 𝟐 = ……… cm3 mol of MnO4– = cV = 0.00500 × 1000 titreaverage = ……….. mol 1 2 final burette reading / cm3 initial burette reading / cm3 volume of FA 2 used / cm3 V1 V2  

4 9729/04/CJC JC2 Preliminary Examination 2019 (ii) Calculate the amount of Fe2+ ions that reacted with the MnO4– ions calculated in (c)(i). amount of Fe2+ ions =………….………....… [ 1] (iii) Calculate the amount of Fe2+ ions in the five crushed tablets, FA 1. amount of Fe2+ ions in FA 1 =………….………....… [1] (iv) Calculate the mass of FeSO4.7H2O in one tablet. [Ar: Fe, 55.8; S, 32.1; O, 16.0; H, 1.0] mass of FeSO4.7H2O in one tablet =………….………....… [ 2] mol of Fe2+ reacted = 5 × mol of MnO4– = 5 × ans to (b)(i) = ……….. mol Mr of FeSO4.7H2O = 55.8 + 32.1 + 4(16.0) + 7[2(1.0) + 16.0] = 277.9 mass of FeSO4.7H2O in 5 tablets = nMr = ans to (b)(iii) x 277.9 = …………. g  mass of FeSO4.7H2O in 1 tablet = 𝟏 𝟓 x above ans = …………. g mol of Fe2+ in 5 crushed tablets = mol of Fe2+ in 250 cm3 solution = 𝟐𝟓𝟎 𝟐𝟓.𝟎 × ans to (b)(ii) = …………. mol

5 9729/04/CJC JC2 Preliminary Examination 2019 (v) Hence calculate the percentage by mass of FeSO4.7H2O in one tablet. percentage by mass of FeSO4.7H2O in one tablet =………….………. [2] (d) Explain why in the procedure described in (a), the volume of sulfuric acid was measured using a measuring cylinder rather than a burette. ……..………………........................................................................................................ …….......................................................................................................................… [1] (e) To confirm the formula of iron(II) sulfate heptahydrate in the tablet, a student dissolved one of the tablets in nitric acid and added a few drops of barium nitrate to the filtrate. State the observation expected from this chemical test. Give your reasoning. ..……………………….................................................................................................... ...............................................................................................................................… [1] [Total: 16] % by mass of FeSO4.7H2O in 1 tablet = 𝟓 × ans to (b)(iv) mass of FA1 x 100 = ………. %  white ppt of BaSO4 formed confirmed the presence of SO42– ion.  sulfuric acid is in excess and so, its volume need not be measured precisely.

6 9729/04/CJC JC2 Preliminary Examination 2019 2 Determination of the enthalpy change for the reaction of citric acid with NaHCO3(aq) Antacid, taken for quick relieve of occasional heartburn, is also readily available in pharmacies. Antacid contains mainly citric acid and sodium hydrogencarbonate, which react when in contact with water to give carbon dioxide. In this experiment , you will determin e the enthalpy change when citric acid reacts with sodium hydrogencarbonate. You are provided with the following. FA 4 is 0.8 mol dm–3 citric acid. FA 5 is solid sodium hydrogencarbonate, NaHCO3. (a) Method You will carry out the following experiment twice.  Weigh between 6.5 g and 7.0 g of FA 5 in a dry weighing bottle.  Use a pipette to transfer 50.0 cm3 of FA 4 into the plastic cup supported in a 250 cm3 beaker.  Place the thermometer in the acid in the plastic cup and record its initial temperature.  Carefully add the weighed sample of FA 5, in small portions, into the acid in the plastic cup. Stir the mixture carefully with the thermometer.  Record the lowest tempera