SNGS 2024 Chem Prelim Paper 3

[Turn over Name: _________________________ ( ) Class: _________ 2024 PRELIMINARY EXAMINATION GENERAL CERTIFICATE OF EDUCATION ORDINARY LEVEL CHEMISTRY 6092/03 Paper 3 Practical 6 August 2024 1h 50 minutes _____________________________________________________________________ READ THESE INSTRUCTIONS FIRST Write your name, register number, 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 a soft pencil for any diagrams or graphs. Do not use highlighters, glue, correction fluid or correction tape. Answer all questions. The use of a scientific calculator is expected, where appropriate. You are reminded of the need for clear presentation in your answers. Qualitative Analysis Notes are printed on page 10. The number of marks is given in brackets [ ] at the end of each question or part question. Shift Laboratory For Examiner’s Use Question 1 (15) Question 2 (11) Question 3 (9) Question 4 (5) Total (40) _____________________________________________________________________ This document consists of 10 printed pages.

2 1 Citric acid is a carboxylic acid found in lemon juice. The equation for the reaction between citric acid, H3C6H5O7, and potassium hydroxide, KOH, is shown below. 3KOH + H3C6H5O7 → K3C6H5O7 + 3H2O The mass of citric acid dissolved in 500 cm3 of an aqueous solution can be determined by titration with aqueous potassium hydroxide. Methyl orange is used to determine the end-point of the titration. Read all the instructions below carefully before starting the experiment in Question 1. P is 0.100 mol/dm3 aqueous potassium hydroxide. Q is aqueous citric acid. (a) Fill the burette with P. Use the pipette to transfer 25.0 cm3 of Q into a conical flask. Add a few drops of the methyl orange to the solution in the conical flask. Add P from the burette, swirling the flask consistently. Record your titration results in the space provided. Repeat the titration as many times as you consider necessary to achieve consistent results. Results [5] (b) Use your titration results to obtain the average volume of P used. Show clearly how you obtained this volume. average volume of P ……………………….[1]

3 (c) Use your results from (b) to calculate the number of moles of aqueous potassium hydroxide in the average volume of P used. moles of aqueous potassium hydroxide ………………………..[1] (d) Calculate the number of moles of citric acid in 25.0 cm3 of Q. moles of citric acid in 25.0 cm3 of Q is ………………………[1] (e) Calculate the concentration of citric acid in Q in mol/dm3. concentration of citric acid in Q .......................................... [1] (f) Calculate the mass of citric acid in 500 cm3 of Q. [Ar : H, 1; C,12; O,16] mass of citric acid in 500 cm3 of Q ........................... [3] (g) If Q was replaced by ethanoic acid of the same concentration during the titration, what would be the expected volume of P required to completely neutralise 25.0 cm3 of Q? Explain your answer. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. ………………………………………………………………………………………...[3] [Total: 15 marks]

4 2 Read all the instructions carefully before starting the experiments in Question 2. You are provided with three compounds of manganese: solid D, solutions S and T. Instructions (a) Carry out the following tests on each substance and record your observations in the table. You should test and name any gas evolved. The volumes given below are approximate and should be estimated rather than measured. test observations Test 1 Put about 1 cm depth of aqueous hydrogen peroxide in a clean test tube. Add an equal volume of dilute sulfuric acid. Then a dd 1 cm3 of T with shaking. You do not need to test any gas evolved in Test 1. Test 2 Put about 1 cm depth of hydrogen peroxide in a clean test tube. Use a spatula to carefully add a small amount of D. Test 3 Put about 1 cm depth of dilute sulfuric acid in a clean test tube. Add an equal volume of aqueous potassium iodide. Use a spatula to carefully add a small amount of D. Test 4 Put about 1 cm depth of S in a clean test tube. A dd aqueous sodium hydroxide until a change is seen. Add excess aqueous sodium hydroxide to the mixture and allow to stand for a few minutes.

5 Test 5 Put about 1 cm depth of S in a clean test tube. A dd an equal volume of aqueous barium nitrate then add an equal volume of dilute nitric acid. Test 6 Put about 1 cm depth of T in a clean test tube. Add an equal volume of S. [9] (b) Deduce the role of D in Test 3. Explain your answer. …………………………………………………………………………………………. …………………………………………………………………………………….......[2] [Total: 11 marks]

6 3 A student carried out a titration to determine the mole ratio for the reaction between acidified hydrogen peroxide and acidified iron(II) sulfate. She followed the procedure below using different volumes of acidified iron(II) sulfate and acidified hydrogen peroxide as shown in Table 3. (i) Put aqueous potassium manganate(VII) into the burette. Titrate 50 cm 3 of 0.050 mol/dm 3 acidified iron(II) sulfate with 0.020 mol/dm 3 aqueous potassium manganate(VII) until one drop of aqueous potassium manganate(VII) produces a pink colour that does not disappear on swirling. (ii) Empty the flask and rinse with water. (iii) Measure 40 cm3 of acidified iron(II) sulfate and 10 cm3 of 0.030 mol/dm3 acidified hydrogen peroxide, using separate measuring cylinders. Pour both solutions into a flask, mix and titrate with aqueous potassium manganate(VII). (iv) Repeat the procedure described in (i)-(iii) using the volumes of acidified iron(II) sulfate and acidified hydrogen peroxide. The results are shown in Table 3. Volume of acidified iron(II) sulfate /cm3 Volume of acidified hydrogen peroxide /cm3 Total volume/cm3 Volume of aqueous potassium manganate(VII)/cm3 50 0 50 24.40 40 10 50 13.60 30 20 50 2.80 20 30 50 8.00 10 40 50 18.80 0 50 50 29.60 Table 3 (a) Plot a graph of volume of aqueous potassium manganate(VII) against volume of acidified iron(II) sulfate on the grid on Page 7. Using these points, draw two straight lines of best fit. The lines should intersect each other.

7 [4] (b) (i) From the graph, deduce the volume of acidified iron(II) sulfate when the lines intersect on the graph. Volume of acidified iron(II) sulfate …………………… cm3[1] (ii) Calculate the volume of acidified hydrogen peroxide that has reacted with the volume of acidified iron(II) sulfate in (b)(i), by using the formula ‘volume of acidified hydrogen peroxide = 50 – volume of acidified iron(II) sulfate’. Volume of acidified hydrogen peroxide ………………… cm3 [1]

8 (c) (i) Calculate the number of moles of iron(II) sulfate in the volume of acidified iron(II) sulfate in (b)(i) Number of moles of iron(II) sulfate = …………………….[1] (ii) Calculate the number of moles of hydrogen peroxide in the volume of acidified hydrogen peroxide in (b)(ii). Number of moles of hydrogen peroxide= …………………….[1] (d) Using your answer to (c), complete the partial equation below. ___