Copy of CHS 2024 S4 OP Prelim Paper 3

Name: Index Number: Class: CATHOLIC HIGH SCHOOL Preliminary Examination Secondary 4 (O-Level Programme) End-of-Year Examination Year 4 (Integrated Programme) CHEMISTRY 6092/03 Paper 3 Practical 2 August 2024 Candidates answer on the Question Paper 1 hour 50 minutes Additional Materials: - READ THESE INSTRUCTIONS FIRST Write your name, index 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 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 page 16. 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 / 18 2 / 11 3 / 7 4 / 4 Total / 40 This document consists of 14 printed pages and 2 blank pages.

2 1 Sodium carbonate can occur as a hydrated solid with the formula Na2CO3∙nH2O. In these crystals, one formula unit of sodium carbonate, Na2CO3, combines with n molecules of water. There are two methods to determine the value of n. Each method involves sodium carbonate reacting with excess hydrochloric acid to release carbon dioxide. Na2CO3∙nH2O(s) + 2HCl(aq) → 2NaCl(aq) + CO2(g) + (n + 1)H2O(l) You will carry out one method each in Experiment 1 and Experiment 2. You will only determine the value of n in hydrated sodium carbonate, Na2CO3∙nH2O, in Experiment 1. Read all the instructions below carefully before starting the experiments in Question 1. Instructions (a) (i) Experiment 1 You will perform a titration to measure the volume of hydrochloric acid that neutralises an aqueous solution of hydrated sodium carbonate, Na2CO3∙nH2O. P is 0.100 mol/dm3 hydrochloric acid. Q is an aqueous so lution containing 14.3 g /dm3 of hydrated sodium carbonate, Na2CO3∙nH2O. R is bromophenol blue indicator. Method Fill the burette with P. Use the pipette to transfer 25.0 cm3 of Q into a conical flask. Add a few drops of R into the flask. Add P from the burette, swirling the flask constantly. At the end-point, one drop of P produces a yellow colour that does not disappear on swirling. Record your titration results in the space provided , repeating the titration as many times as you consider necessary to achieve consistent results. Results [5]

3 [Turn over (ii) Use your titration results to obtain the average volume of P used. Show clearly how you obtained this volume. average volume of P = ……………………. cm3 [1] (b) (i) Calculate the amount, in moles, of hydrochloric acid present in the average volume of P. amount of hydrochloric acid = …….…………. mol [1] (ii) Use the equation to deduce the amount, in mol es, of Na2CO3 present in 25.0 cm3 of Na2CO3∙nH2O. amount of Na2CO3 = …………………. mol [1] (iii) Hence, calculate the amount, in mol es, of Na2CO3 present in 1.00 dm 3 of Na2CO3∙nH2O. amount of Na2CO3 in 1.00 dm3 = …………………. mol [1] (iv) Calculate the value of n in the sample of Na2CO3∙nH2O. [Ar: C, 12; H, 1; O, 16; Na, 23] n = …………………. [3]

4 (c) The pH range over which bromophenol blue changes colour is 3.0 to 4.6. The pH range over which phenolphthalein changes colour is 8.0 to 9.8. A student repeated Experiment 1 using phenolphthalein instead of bromophenol blue. State and explain the effect, if any, on the student’s calculated value of n. …………………………………………………………………………………………………....... …………………………………………………………………………………………………....... …………………………………………………………………………………………………....... …………………………………………………………………………………………………....... ….…………………………………………………………………………………………….... [2]

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6 (d) Experiment 2 You will measure the volume of carbon dioxide released when hydrated sodium carbonate, Na2CO3∙nH2O, reacts with excess hydrochloric acid. X is 2.0 mol/dm3 hydrochloric acid, HCl. Y is hydrated sodium carbonate, Na2CO3∙nH2O in solid form. Method • Fill the tub with tap water to approximately two-third depth. • Fill the 100 cm 3 measuring cylinder completely with tap water. Using your palm to firmly cover the top of the measuring cylinder, invert the measuring cylinder and place it in the water in the tub. • Remove your palm. Check that there are no air bubbles in the measuring cylinder and clamp the measuring cylinder so the open end is just below the surface of the water. • Using the 50 cm3 measuring cylinder, transfer 35 cm3 of X into a boiling tube. Check that the rubber bung fits tightly into the mouth of the boiling tube and place the end of the rubber tubing into the inverted 100 cm3 measuring cylinder. • Weigh the plastic container and Y and record the mass. • Remove the rubber bung from the boiling tube. Carefully add Y from the container into the boiling tube and replace the rubber bung immediately. Swirl the boiling tube. • Allow the reaction to come to completion. Record the volume of gas collected. • Weigh the empty plastic container and record the mass. • Calculate the mass of Y used in the reaction. • You may begin the experiments for Question 2 on page 8 while you are waiting for the reaction to complete. inverted 100 cm3 measuring cylinder tub rubber bung rubber tubing retort stand and clamp tap water

7 [Turn over Results mass of container and Y / g mass of empty container / g mass of Y used / g volume of gas / cm3 [2] (e) (i) The volume of gas obtained in Experiment 2 is smaller than expected. Other than solubility of carbon dioxide gas, suggest the main source of error in the experiment. ………………………………………………………...…………………………………….. ….………………………………………………………..…………………………….... [1] (ii) Suggest an improvement you could make to the experiment to reduce this source of error. ………………………………………………………...…………………………………….. ….………………………………………………………..…………………………….... [1] [Total: 18]

8 2 You will carry out reactions using two compounds of manganese. T is solid potassium manganate(VII), KMnO4. S is an aqueous manganese(II) salt. Read all the instructions carefully before starting the experiments in Question 2. Instructions (a) Carefully transfer half a spatula of T to a clean and dry test-tube. Heat the sample in the test-tube strongly for about 1 minute. After heating, leave the test-tube to cool and keep it for Test 1 in (b)(i). You may wish to start Test 2 in (b)(i) while you wait. Record all your observations including testing the gas produced. ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ………………………………………………………………………………………………...… [2] (b) (i) Carry out the following tests on T and S. Test and identify any gases evolved. Record your observations in the table. If no change is observed for a test, write ‘no observable changes’. The volumes given below are approximate and should be estimated rather than measured. test instructions observations 1 To the cooled test-tube from (a), add distilled water, a little at a time, to a depth of approximately 4 cm . Shake the test-tube and then leave the c