CJC 2022 JC2 H2 Chem Prelim P4 Answers for Review

[Turn over 9729/04/CJC JC2 Preliminary Examinations 2022 CANDIDATE NAME CLASS 2T CHEMISTRY 9729/04 Paper 4 Practical 23 August 2022 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 20 and 21. 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 24 printed pages and 0 blank page. Catholic Junior College JC2 Preliminary Examinations Higher 2 Shift Laboratory For Examiner’s Use 1 / 15 2 / 13 3 / 27 Total / 55 ANSWERS for review

2 [Turn over 9729/04/CJC JC2 Preliminary Examinations 2022 Answer all the questions in the spaces provided. 1 Determination of the Mr of a hydrated ethanedioate salt , using acidified potassium manganate(VII) by titration Calcium ethanedioate is the major component of the most common type of human kidney stones. It is one of a series of salts formed from ethanedioic acid, H 2C2O4. Another of these salts can be represented by the formula X2C2O4H2O, where X is a Group 1 metal. Solution Q contains 64.5 g dm–3 of X2C2O4H2O in deionised water. You are not provided with Q. FA 1 is a diluted solution of Q, in which 35.70 cm 3 of Q was made up to 250 cm 3 with deionised water in a graduated flask. FA 2 is 0.0200 mol dm–3 potassium manganate(VII), KMnO4. FA 3 is 1.00 mol dm–3 sulfuric acid, H2SO4. In this question, you will perform a titration. The data from this titration will be used to determine:  the concentration of C2O42– ions in Q.  the Mr of X2C2O4H2O, and hence the identity of the metal X. (a) Titration of FA 1 against FA 2 In this titration, FA 2 is run from a burette into the conical flask containing FA 1 and FA 3. Initially, the colour of the FA 2 will take some time to disappear. After some FA 2 has been added, sufficient Mn 2+(aq) ions will be present to allow the reaction to occur faster. The end point is reached when a permanent pale pink colour is obtained. (i) 1. Fill the burette with FA 2. 2. Using a pipette, transfer 25.0 cm3 of FA 1 into the conical flask. 3. Using an appropriate measuring cylinder, transfer 25.0 cm3 of FA 3 to the same conical flask. 4. Heat this solution to about 60 oC. 5. Run FA 2 from the burette into this flask until a permanent pale pink colour is obtained. Be careful when you titrate under the hot condition. 6. Record your titration results to an appropriate level of precision, in the space provided next page. 7. Repeat points 1 to 6 until consistent results are obtained. 8. Turn off your Bunsen burner.

3 [Turn over 9729/04/CJC JC2 Preliminary Examinations 2022 Titration results 1 2 Final burette reading /cm3 24.95 49.90 Initial burette reading /cm3 0.00 24.95 Volume of FA 2 used /cm3 24.95 24.95  Tabulate initial and final burette readings and volume used in the titration table and table has correct headers and units.  Consistent burette readings taken (VFA 2 ± 0.10 cm3) and recorded to 2 d.p.  Accuracy of titration results. [5] (ii) From your titrations, obtain a suitable volume of FA 2 to be used in your calculations. Show clearly how you obtained this volume. Average volume of FA 2 = ( ૛૝.ૢ૞ା૛૝.ૢ૞ ૛ ) = 24.95 cm3 volume of FA 2 = …………………………..[1] (iii) The equation for the reaction between ethanedioate ions , C 2O42– and manganate(VII) ions is shown below. 5C2O42–(aq) + 2MnO4–(aq) + 16H+(aq) → 2Mn2+(aq) + 8H2O(l) + 10CO2(g) Calculate the amount of ethanedioate ions, C2O42– in 25.0 cm3 of FA 1. Amount of MnO4– = ( ૛૝.ૢ૞ ૚૙૙૙ x 0.0200) = 4.99 x 10–4 mol 5C2O4 2– ≡ 2MnO4– Amount of C2O42– in 25.0 cm3 of FA 1 = ( ૞ ૛ x 0.000499) = 1.25 x 10–3 mol amount of C2O42– in in 25.0 cm3 of FA 1 = …………………………..[2] 24.95 cm3 24.95 cm3 24.95 cm3 1.25 x 10–3 mol [Q] = 64.5 g dm–3 of X2C2O4H2O in deionised water 37.50cm3 25.0 cm3 dilution Q diluted Q FA1 0.0200 mol dm–3 potassium manganate(VII), KMnO4 FA2 250 cm3 Took out 1/10 portion

4 [Turn over 9729/04/CJC JC2 Preliminary Examinations 2022 (iv) Determine the concentration in mol dm –3, of C 2O42– in Q. (concentrated solution) Amount of C2O4 2– in 250 cm3 of FA 1 = ( ૛૞૙ ૛૞.૙ x 1.25 x 10–3) mol = (10 x 1.25 x 10–3) mol = 0.0125 mol Amount of C2O4 2– present in 35.70 cm3 of Q = 0.0125 mol Concentration of C2O42– in Q = ( ૚૙૙૙ ૜૞.ૠ૙ x 0.0125) mol = 0.349 mol dm–3 concentration of C2O42– in Q = …………………………..[2] (v) Use your answer to (a)(iv) to calculate the Mr of the ethanedioate salt. Given: solution Q contains 64.5 g dm–3 of X2C2O4H2O. Mr of X2C2O4H2O = ૟૝.૞ ૙.૜૝ૢ = 184.8 Mr of the ethanedioate salt = …………………………..[1] (vi) Hence, deduce the identity of X. Show your working clearly. [Ar: C, 12.0; O, 16.0; H, 1.0; Li, 6.9; Na, 23.0; K, 39.1; Rb, 85.5; Cs, 132.9; Fr, 223.0] Mr of X2C2O4H2O = 184.8 2(Ar of X) + 88.0 + 18.0 = 184.8 Ar of X = 39.4 So, X is K or potassium X is …………………………..[2] (b) A student performed the experiment in (a)(i) using a sample of another ethanedioate salt. The student obtained a mean titre value of 22.20 cm3. The teacher calculated that the volume of FA 2 required should be 22.40 cm 3. The teacher told the student that the total percentage error from the apparatus in the experiment was 0.4%. Calculate the error in the student’s result, based on these data. State and explain whether or not the student’s result is accurate. ……………………………………………………………………………………………… . ……………………………………………………………………………………………… . ……………………………………………………………………………………………… . ……………………………………………………………………………………………… . …………………………………………………………………………………………… .[2] [Total: 15] 0.349 mol dm–3 184.8 K or potassium Experimental error = ( ૛૛.૝૙ି૛૛.૛૙ ૛૛.૝૙ ) x 100% = 0.893% Candidate states that experiment is not accurate and justifies this in terms of experimental error being > apparatus error.

5 [Turn over 9729/04/CJC JC2 Preliminary Examinations 2022 2 Investigation of reaction between manganate(VII) ions