NJC_2020 Prelim Paper 4_Suggested Answer

1 NJC SH2 Preliminary Examination 9729/04/20 NATIONAL JUNIOR COLLEGE SH 2 Year − End Practical Examination Higher 2 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 4 Practical Candidate answer on the Question paper. 9729/04 Wednesday 19 August 2020 2 hours 30 minutes READ THESE INSTRUCTIONS FIRST Write your identification number and name. Give details of the practical shift and laboratory where appropriate, in the boxes provided. Write in 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 19 and 20. The number of marks is given in brackets [ ] at the end of each question or part question. This paper consists of 18 printed pages including this cover page. Shift Laboratory For Examiner’s use 1 / 16 2 / 13 3 / 13 4 / 12 Presentation / 1 Total / 55

2 NJC SH2 Preliminary Examination 9729/04/20 1 Determination of a value for the number of water of crystallization, x, in an iron(II) salt, (NH4)2Fe(SO4)2. x H2O. The iron (II) salt, (NH4)2Fe(SO4)2. x H2O, can be titrated against potassium manganate (VII) solution. The two half–equations for the reaction are shown below. MnO4 + 8H+ + 5e− Mn2+ + 4H2O Fe2+ Fe3+ + e− FA 1 is potassium manganate(VII), KMnO4 solution, of concentration around 0.02 mol dm–3 FA 2 is 0.0780 mol dm–3 ethanedioate ions C2O42– FA 3 is a solution of the iron(II) salt containing 38.4 g dm–3 of (NH4)2Fe(SO4)2. x H2O FA 4 is dilute sulfuric acid, H2SO4 The exact concentration of FA 1 can be determined when FA 2 is titrated against it. C2O42 reacts with MnO4 in an acidic medium to form Mn2+ and CO2 gas. 2MnO4 + 16H+ + 5C2O42 2Mn2+ + 8H2O + 10CO2 As this reaction is very slow at the start , the C2O42 solution is heated prior to titration against MnO4. However, it is not necessary to continuously heat the solution as the rate of reaction will increase as the reaction proceeds. Method (a) (i) Standardisation of FA 1 solution 1. Fill the burette with FA 1. 2. Using a pipette, transfer 10.0 cm3 of FA 2 into the conical flask. 3. Using an appropriate measuring cylinder, transfer 20 cm 3 of FA 4 to the same conical flask. 4. Place a 1 oC thermometer into this conical flask. 5. Heat the contents of the conical flask using the bunsen burner. 6. Turn off the Bunsen burner once a t