2020 SAJC H2 Prelim Paper 4 (Answers)

[Turn Over NAME Class ST ANDREW’S JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATION Chemistry (9729) Paper 4 Practical 17 August 2020 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 on all the work you hand in. Give details of the practical shift and laboratory 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 the 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. Shift Laboratory Qualitative Analysis Notes are printed on pages 21 and 22. The number of marks is given in the brackets [ ] at the end of each question or part question. For Examiners’ Use 1 2 3 4 Total This document consists of 22 printed pages.

2 1 Investigation of the concentration of potassium manganate(VII) Potassium manganate(VII) is a very useful oxidising agent. It oxidises iron( II) ions in acidic medium according to equation 1. MnO4 (aq) + 5Fe2+ (aq) + 8H+ (aq)  Mn2+ (aq) + 5Fe3+ (aq) + 4H2O (l) equation 1 In solution, ammonium iron (II) sulfate, (NH 4)2SO4FeSO46H2O, dissociates into iron (II), sulfate and ammonium ions according to equation 2. (NH4)2SO4FeSO46H2O (s)  2NH4+ (aq) + 2SO42 (aq) + Fe2+ (aq) + 6H2O (l) equation 2 You are provided with FA 1, ammonium iron(II) sulfate crystals, FA 2, potassium manganate(VII) solution, 1.0 mol dm3 sulfuric acid. (a) (i) Preparation of standard solution of FA 3 and titration of FA 3 against FA 2 In an appropriate format in the space provided on page 3, prepare tables in which to record results for your experiment in (a):  All weighings to an appropriate level of precision  All burette readings and the volume of FA 2 added 1. Weigh the capped bottle containing FA 1. 2. Dissolve this in about 50 cm3 of 1.0 mol dm-3 sulfuric acid in a 100 cm3 beaker. 3. Transfer the solution and washings into a 250 cm3 standard flask. Make up to the mark with deionised water and shake well to obtain a homogeneous solution, FA 3. 4. Reweigh the emptied bottle and its cap. 5. Fill a burette with FA 2. 6. Pipette 25.0 cm3 of FA 3 prepared into a conical flask. 7. Add about 10 cm3 of aqueous sulfuric acid into the conical flask. 8. Run FA 2 from the burette into the flask. The end -point is reached when the solutions changes from pale green to orange. 9. Repeat steps 6 to 8 until consistent titres are obtained.

3 [Turn Over Results Mass of weighing bottle + FA1 /g 15.35 Mass of emptied weighing bottle /g 6.16 Mass of FA1 /g 9.19 Burette readings 1 2 Final burette reading / cm3 23.50 47.00 Initial burette reading / cm3 0.00 23.50 Volume of FA 2 / cm3 23.50 23.50 Values used √ √ [4]  correct headers and units  2 dp for mass (according to mass balance)  2 dp for burette readings and volume of FA 2  2 consistent titre readings within +/- 0.10 cm3 (ii) From your titrations, obtain a suitable volume of FA 2 to be used in your calculations. Show clearly how you obtained this volume. (23.50 + 23.50)/2 = 23.50 cm3 (2dp) Volume of FA 2 = ………………………………… [1] (b) [Ar of K: 39.1; Mn: 54.9; O: 16.0; N: 14.0; H: 1.0; Fe: 55.8; S:32.1] (i) Calculate the number of moles of FA 1 in the weighed sample and hence calculate the number of moles of Fe2+ ions used during the titration. No. of moles of (NH4)2SO4  FeSO4  6H2O = (mass of FA 1) /392 = 0.02344 mol No. of moles of Fe2+ in 250 cm3 = 0.02344 mol No. of moles of Fe2+ used during titration = 0.02344 x (25/250) = 0.00234 mol

4 number of moles of FA 1 = ……………………………… number of moles of Fe2+ ions used = ……………………………… [2] (ii) Calculate the number of moles of MnO 4− reacted during titration and hence the mass of potassium manganate (VII), KMnO4, in 1 dm3 of FA 2. No. of moles of MnO4¯ reacted = (b)(i) / 5 = 4.688 X 10-4 mol Concentration of MnO4¯ reacted = 4.688 × 10-4 (a)(ii) = 0.01995 mol dm-3 Mass of KMnO4 reacted in 1 dm3 = 0.01995 X 158 = 3.15 g number of moles of MnO4− reacted = …………………………… mass of KMnO4 in FA 2 = …………………………… [3] (c) A student performed the same experiment, using the quantities described earlier, and obtained a mean titre volume of 25.45 cm3. The errors (uncertainties) associated with each reading is shown below: Apparatus Uncertainties volumetric flask ±0.15 cm3 pipette ±0.1 cm3 burette ±0.05 cm3 Calculate the maximum percentage error (uncertainty) of this mean titre volume. Percentage error of volumetric flask = Percentage error of pipette = Error in titre (2 burette readings) = 2 X 0.05 = 0.1 cm3 Percentage error of burette = Total error = (0.06 + 0.4 +0.39) = 0.85 % maximum percentage error = …………………………..% [2] %060.0250 10015.0  %40.00.25 10010.0    %39.045.25 1001.0   

5 [Turn Over (d) The teacher informed the student who performed the experiment in (c) that the titre volume should have been 26.00 cm3 instead. (i) Calculate the percentage error in the student’s result. Difference in titre volume = 26.00 – 25.45 = 0.55 cm3 Percentage error in the student’s result = 0.55 / 26.00 x 100% = 2.12% (3 s.f.) percentage error = ………………………..% [1] (ii) Using your answers in (c) and (d)(i), comment on the accuracy of the student’s result. Since the percentage error is larger than the apparatus error, the student’s result is not accurate. [1] [Total: 14]

6 2 Investigation of the effect of temperature on chemical reactions Potassium manganate(VII) is widely used as a strong oxidising agent. It can oxidise both organic and inorganic compounds. Besides oxidising iron( II) ions, it can also oxidise ethanedioate ions. 2MnO4− (aq) + 5C2O42− (aq) + 16H+ (aq)  2Mn2+ (aq) + 10CO2 (g) + 8H2O (l) equation 3 In this experiment, the effect of temperature on the rate of this reaction is studied by measuring the time taken for the purple solution to turn colourless. You will perform a series of four experiments. Then, you will graphically analyse your results to determine the effect of temperature on the rate of the reaction between potassium manganate(VII) and ethanedioate ions. For each experiment, you will note the time taken, t, for the reaction mixture to become colourless. You will use the same potassium m anganate(VII) solution, FA 2, and 1.0 mol dm −3 sulfuric acid, H2SO4, in Question 1. You are also provided with FA 4, 0.1 mol dm−3 sodium ethanedioate, Na2C2O4. (a) Prepare a tabl e in the space provided on page 9 in which to record for each experiment, with the appropriate level of precision:  temperature of the solution inside the boiling tube,  time taken for solution to turn colourless, t, to the nearest second,  calculated values of 1/t and lg (1/t). It is recommended for you to draw this table after step 6 of experiment 1 on page 8. Safety notes Hot water to be collected from the metal dispenser is heated to 90 °C. Use a Styrofoam cup placed in a 250 cm3 beaker to collect the hot water (according to step 4 of experiment 1 on the next page) to prevent your hands from being scalded. Gently pull/push the tap of the dispenser to prevent the hot water from spurting/splashing out.

7 [Turn Over Experiment 1 1. Using a measuring cylinder, add 5.0 cm3 of FA 2 to a boiling tube. 2. Using the same measuring cylinder, add 10.0 cm3 of 1.0 mol dm−3 H2SO4(aq) to the boiling tube. 3. Using another measuring cylinder, measure 5.0 cm3 of FA 4. Do not add it to the boiling tube yet. 4. Place a Styro