Copy of MGS Chemistry P3 (Q1 and Q2)

Page 2 of 11 Methodist Girls’ School Chemistry Paper 3 Sec 4 Preliminary Examination 2024 1 P is a solution that contains 0. 0164 mol of KIO3, in 1.00 dm 3 of water. An acidified solution of KIO3 oxidises potassium iodide to iodine. KIO3 + 5 I- + 6 H+ → K+ + 3 I2 + 3 H2O The iodine produced can be titrated with Q, a solution of an organic acid. 1 mole of iodine reacts with 1 mole of the organic acid. You are going to determine the relative molecular mass of the organic acid by using titration. Q was prepared by dissolving 10.0 g of the solid organic acid, R, in 1.00 dm 3 of water. Read all the instructions carefully before starting the experiments. Instructions (a) Put Q into the burette. Pipette 25.0 cm 3 of P into a flask. Add 1 test-tube full of aqueous potassium iodide solution and one test-tube full of dilute sulfuric acid to P in the flask. Swirl to mix the solutions well. Add Q from the burette until the red-brown colour fades to pale yellow, then add a few drops of the starch solution. This will give a deep blue, almost black colour with a small quantity of iodine remaining. Continue adding Q slowly from the burette until one drop of Q causes the blue colour to disappear, leaving a colourless solution. Record your titration results in the space provided, repeating the titration as many times as you consider necessary to achieve consistent results. Results [5]

Page 3 of 11 Methodist Girls’ School Chemistry Paper 3 Sec 4 Preliminary Examination 2024 (b) From your titration results, obtain an average volume of Q to be used in your calculations. Show clearly how you obtained this volume. Average volume of Q………………………[1] (c) The equation show s that one mole of KIO3 reacts with 5 moles of potassium iodide to produce 3 moles of iodine. Calculate the number of moles of iodine produced. Number of moles of iodine produced …..………[2] (d) From your answer in (c), calculate the number of moles of the organic acid , R, in Q that reacted with the iodine. Number of moles of organic acid that reacted with iodine …..………[1] (e) Using relevant information from the question and your answer in (d), determine the relative molecular mass of the organic acid, R, in Q. Show all relevant working. Relative Molecular mass of organic acid, R …..………[2] [Total: 11]

Page 4 of 11 Methodist Girls’ School Chemistry Paper 3 Sec 4 Preliminary Examination 2024 2 Five experiments were conducted by a student to investigate the temperature change when another organic acid, C, dissolves in water. The relative molecular mass of C is 192. Experiment 1 Measure 200.0 cm3 of deionised water using a measuring cylinder and pour it into a Styrofoam cup, nested in a beaker. Add 0.04 moles of solid C into the deionised water , stir with the thermometer and measure the temperature until no further change. Record the temperature change. Empty the Styrofoam cup and rinse it with water. Experiments 2 to 5 Repeat Experiment 1 but use different number of moles of solid C for each experiment. The results from the students’ investigation are shown in the table. Results experiment no. of moles of solid C temperature change / oC 1 0.04 - 1.0 2 0.08 - 3.0 3 0.12 - 5.0 4 0.16 - 7.0 5 0.2 - 9.0

Page 5 of 11 Methodist Girls’ School Chemistry Paper 3 Sec 4 Preliminary Examination 2024 (a) Plot the results on the grid with number of moles of the organic acid, C, against temperature change. Complete the graph by drawing a line of best fit. [4]

Page 6 of 11 Methodist Girls’ School Chemistry Paper 3 Sec 4 Preliminary Examination 2024 (b) Describe the trend shown by your graph in (a). [1] (c) From the graph, determine the temperature change when 9.60 g of solid C is dissolved in 200.0 cm3 of deionised water. Temperature change is ………………[1] (d) You are going to conduct Experiment 6. Measure 9.60 g of solid C and put into a Styrofoam cup, nested in a beaker. Add 200.0 cm3 of deionised water into it. Stir with the thermometer to dissolve solid C until no further change in temperature. Record all necessary data to clearly show all necessary workings on how the temperature change is obtained. (Mr of C = 192) [2] (e) Explain why the temperature change obtained from the graph in (c) is more reliable. [1]

Page 7 of 11 Methodist Girls’ School Chemistry Paper 3 Sec 4 Preliminary Examination 2024 (f) In another experiment, the solubility of the organic acid , C, in different solvents, water, ethanol and acetone is investigated. Plan an investigation to find the effect of the nature of solvent on the solubility of the organic acid, C. You may assume the apparatus normally found in the school laboratory is available for the investigation. [5] [Total: 14]