Cambridge A Level Chemistry 9701 — 2020 May/June Paper 3 · Variant 1

1 In this experiment you will carry out a titration to determine the relative formula mass of a hydrated salt, FA 1. FA 1 is a hydrated salt. FA 2 is dilute sulfuric acid. FA 3 is 0.0200 mol dm–3 potassium manganate(VII). (a) Method Preparing a solution of FA 1 ●● Weigh the stoppered container of FA 1. Record the mass in the space below. ●● Tip all the FA 1 into the beaker. ●● Reweigh the container with its stopper. Record the mass. ●● Calculate and record the mass of FA 1 used. ●● Add approximately 100 cm3 of FA 2 to the FA 1 in the beaker. ●● Stir the mixture until all the FA 1 has dissolved. ●● Transfer this solution into the 250 cm3 volumetric flask. ●● Rinse the beaker and glass rod with distilled water and transfer the washings to the volumetric flask. ●● Make up the solution in the volumetric flask to the mark using distilled water. ●● Shake the flask thoroughly. ●● This solution of the hydrated salt is FA 4. Label the flask FA 4. Titration ●● Fill the burette with FA 3. ●● Pipette 25.0 cm3 of FA 4 into a conical flask. ●● Use the 25.0 cm3 measuring cylinder to add 10 cm3 of FA 2 to the FA 4 in the conical flask. ●● Perform a rough titration and record your burette readings in the space below. The rough titre is .............................. cm3. ●● Carry out as many accurate titrations as you think necessary to obtain consistent results. ●● Make sure any recorded results show the precision of your practical work. ●● Record in a suitable form below all of your burette readings and the volume of FA 3 added I in each accurate titration. I Keep FA 3 and FA 4 for use in Question 3. II IV V VI VII VI I [8] (b) From your accurate titration results, obtain a suitable value for the volume of FA 3 to be used in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FA 4 required .............................. cm3 of FA 3. [1] (c) Calculations (i) Calculate the number of moles of potassium manganate(VII) present in the volume of FA 3 calculated in (b). moles of KMnO4 = .............................. mol [1] (ii) 1 mol of KMnO4 reacts with 5 mol of the hydrated salt, FA 1. Calculate the concentration of the hydrated salt, in mol dm–3, in FA 4. concentration of FA 4 = .............................. mol dm–3 [1] (iii) Use your answer to (c)(ii), and your data on page 2, to calculate an experimentally determined value for the relative formula mass of the hydrated salt, FA 1. Show your working. Mr of FA 1 = .............................. [1] [Total: 12]

Mark scheme: 1(a) I The following headings and data are recorded in the space provided • mass of container with FA 1 • mass of (empty) container • mass of FA 1 used, correctly subtracted • consistent decimal places for weighings (at least one d.p.) 1 II All the following data is recorded • two burette readings and titre for the rough titration • initial and final burette readings for two (or more) accurate titrations 1 III Titre values recorded for accurate titrations, and Appropriate headings and units in the accurate titration table • initial / start (burette) reading / volume • final / end (burette) reading / volume • titre or volume used / added / or FA 1 added (not ‘difference’ or ‘total’ or ‘amount’) • unit: / cm3 or (cm3) or in cm3 (for each heading) or cm3 unit given for each volume recorded 1 IV All accurate burette readings are recorded to the nearest 0.05 cm3. 1 V The final accurate titre recorded is within 0.10 cm3 of any other accurate titre 1 Award VI, VII and VIII if δ ⩽ 0.020 (cm3 g–1) Award VI and VII if 0.020 < δ ⩽ 0.040 Award VI, only if 0.040 < δ ⩽ 0.060 3 1(b) Candidate calculates the mean correctly. • Candidate must take the average of two (or more) titres that are within a total spread of not more than 0.20 cm3. • Working/explanation must be shown or ticks must be put next to the two (or more) accurate readings selected. • The mean should be quoted to 2 dp and be rounded to nearest 0.01 cm3. 1 1(c)(i) Correct calculation (expressed to 3 or 4 sf) No of moles of KMnO4 used = 0.0200 × mean titre / 1000 The candidate’s mean titre must be used in the calculation 1 Question Answer Marks 1(c)(ii) Correct use of (i) to calculate concentration of FA 4 Concentration of FA 4 = ans (i) × 5 × 1000 / 25 1 1(c)(iii) Correct expression for Mr Mr = mass of FA 1 used × 4 / answer (ii) alternatively: Mr = ans (i) / 4 1

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2 In this experiment you will determine the enthalpy change of solution for anhydrous sodium carbonate. FA 5 is anhydrous sodium carbonate, Na2CO3. (You are given approximately 11 g.) (a) Method Experiment 1 ●● Weigh a cup. Record the mass. ●● Transfer 4.0– 4.2 g of FA 5 from the container into the cup. ●● Reweigh and record the mass of the cup with FA 5. ●● Calculate and record the mass of FA 5 used. ●● Support the cup in the 250 cm3 beaker. ●● Pour 30 cm3 of distilled water into the 50 cm3 measuring cylinder. ●● Measure and record the temperature of the distilled water in the measuring cylinder. ●● Add the 30 cm3 of distilled water to the FA 5 in the cup. ●● Stir constantly until the maximum temperature is reached. ●● Measure and record the maximum temperature. ●● Calculate and record the temperature rise. Experiment 2 ●● Repeat Experiment 1 but this time use 5.0–5.2 g of FA 5 and the other cup. ●● Record all data from both experiments in one table. I II III IV [4] (b) Calculations (i) Calculate the energy produced during Experiment 1. (Assume that 4.2 J change the temperature of 1.0 cm3 of solution by 1.0 °C.) energy produced = .............................. J [1] (ii) Calculate the number of moles of Na2CO3 used in Experiment 1. moles of Na2CO3 = .............................. mol [1] (iii) Use your answers to (b)(i) and (b)(ii) to calculate the enthalpy change, in kJ mol–1, for the reaction below. Show your working. Na2CO3(s) + aq → Na2CO3(aq) enthalpy change = ...... ............................. kJ mol–1 sign value [1] (c) (i) A student suggested that by using the same thermometer, quantities of FA 5, and water, a more accurate value for the temperature rise could be calculated. Suggest how the student could obtain a more accurate measurement. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [1] (ii) State the maximum error in a single thermometer reading in your experiment in (a). maximum error = .............................. Hence calculate the maximum percentage error in the measurement of the temperature rise in Experiment 2. % error = .............................. [2] [Total: 10] Qualitative Analysis Where reagents are selected for use in a test, the name or correct formula of the element or compound must be given. At each stage of any test you are to record details of the following: ● colour changes seen ● the formation of any precipitate and its solubility in an excess of the reagent added ● the formation of any gas and its identification by a suitable test. You should indicate clearly at what stage in a test a change occurs. If any solution is warmed, a boiling tube must be used. Rinse and reuse test-tubes and boiling tubes where possible. No additional tests for ions present should be attempted.

Mark scheme: 2(a) I Unambiguous table of data • Mass of cup • Mass of cup + FA 5 • Mass of FA 5 used • Initial temperature or temperature of water / °C • Final / maximum temperature / °C • Temperature rise / °C • Both experiments must be attempted 1 II Readings recorded appropriately • All four thermometer readings recorded to .0 or .5. • All four subtractions correct (two masses and two temp rises). • No thermometer reading is less than 10 °C • Mass of FA 5 used for first experiment is 4.0–4.2 g • Mass of FA 5 used for second experiment is 5.0–5.2 g 1 III + IV Accuracy marks Check and correct temp subtractions for candidate and supervisor. Round each measured temperature to 0.5 °C if necessary Compare the temperature rises for the two experiments. Award III if candidate’s rise is within 1.0 °C of supervisor in Expt 1. 1 Award IV if candidate’s rise within 1.0 °C of supervisor in Expt 2. 1 2(b)(i) Correct calculation of energy change (2, 3 or 4 sf) Energy change = 30 × 4.2 × temp rise (J) 1 2(b)(ii) Correct calculation of no of moles of Na2CO3 used (2–4 sf) n = mass claimed / 106 1 2(b)(iii) Correct use of (i) and (ii) to calculate enthalpy change (2–4 sf) Enthalpy change = – energy / 1000 × moles 1 Question Answer Marks 2(c)(i) Accuracy improvement Plot graph to obtain better (estimate of) temp rise or plot a cooling curve 1 2(c)(ii) Error = 0.5 1 Correct calculation of % error to 2 or more sig fig % error = 2 × 0.5 / temp rise (expt 2) × 100 1

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3 (a) FA 6 is a hydrated salt. It contains two cations and one anion, all of which are listed in the Qualitative Analysis Notes. (i) Describe and carry out tests to identify the cations in FA 6. Record your tests and observations in the space below. The cations in FA 6 are ............................................... and ���������������������������������������������� . [5] (ii) The anion in FA 6 is a sulfite, sulfate or a halide. Carry out a test to identify the anion in FA 6. Record your tests and observations in the space below. The anion in FA 6 is .................................. . [2] (iii) Give the ionic equation for one reaction you have carried out in (a)(i) or (a)(ii). Include state symbols. ....................................................................................................................................... [1] (iv) The formula of FA 6 is XY2Z2•wH2O where ●● X and Y are the cations present and Z is the anion present ●● w is the number of moles of water of crystallisation in the hydrated salt. The relative formula mass of this compound is 392.0. Using your conclusions from (a)(i) and (a)(ii), calculate the value of w, the number of moles of water of crystallisation. w = .............................. [2] (b) FA 7 and FA 8 are aqueous solutions of covalently bonded compounds. Half fill the beaker with water and place it on a tripod and gauze. Heat until the water begins to boil and then turn off the Bunsen burner. This will be used as a hot water bath. (i) Complete the table by carrying out the tests described. Use a 1 cm depth of FA 7 or FA 8 in a test-tube for each test. observation(s) test FA 7 FA 8 Test 1 Add an equal volume of dilute sulfuric acid and a few drops of FA 3, aqueous acidified potassium manganate(VII), then place in the hot water bath for several minutes. Test 2 Add an equal volume of dilute sulfuric acid and an equal volume of aqueous potassium iodide, then add a few drops of aqueous starch. Test 3 Add an equal volume of aqueous iodine, then add aqueous sodium hydroxide until no further change occurs. Leave the tube to stand. Test 4 Add a few drops of FA 4, then add aqueous ammonia. [5]

Mark scheme: 3(a)(i) Reagents used are NaOH and NH3 1 FA 6 dissolved in (distilled) water (before carrying out tests) 1 Observations with both cold alkalis • With NaOH: green ppt, insoluble in excess • With NH3: green ppt, insoluble in excess OR • If only one of NaOH or NH3 was selected, award this mark if the observation is correct, but it must include ‘ppt turns brown’. 1 Observation when heated with NaOH Fizzing/bubbling and gas/NH3 turns (moist red) litmus to blue 1 Both ions correctly identified Iron(II) and ammonium (Fe2+ and NH4+) 1 Question Answer Marks 3(a)(ii) Anion test and first observation • Add barium nitrate/chloride • White precipitate 1 Observation with acid and conclusion: • white ppt is insoluble in specified mineral acid (not H2SO4) • sulfate / SO42– present 1 3(a)(iii) Ionic equation Any one of the following equations, provided that the appropriate test was carried out. • Fe2+(aq) + 2OH–(aq) → Fe(OH)2(s) • NH4+(aq) + OH–(aq) → NH3(g) + H2O(l or g) • Ba2+(aq) + SO42–(aq) → BaSO4(s) 1 3(a)(iv) Correct use of Mr to calculate no of moles water. Mass of water = (392) - 55.8 -192.2 – 36 1 • n(H2O) = 392 – 284/18 (expressed as integer) 1 Question Answer Marks 3(b)(i) Award one mark for every two correct observations (*) as shown in table below 5 test observation(s) FA 5 FA 6 Test 1 KMnO4 decolorised*. fizzing/bubbling/effervescence or gas relights glowing spill* ......................................................................... No (further) change no reaction KMnO4 not decolorised and ......................................................................... KMnO4 goes from purple to colourless (solution)* Test 2 Red-brown/ brown solution formed* ......................................................................... Mixture goes to (dark) blue / blue-black* No change and ......................................................................... No change* Test 3 Yellow or colourless solution formed* (On standing) off-white / pale yellow precipitate formed*. Test 4 No change / pale yellow (solution) formed* ......................................................................... Red-brown / brown / rust precipitate (formed)* Ignore fizzing 3(b)(ii) FA 8 is ethanol or propan-2-ol or butan-2-ol (or any secondary 2-ol) Correct reference to the CHI3 test or to CH3CH(OH)- or CH3CO group 1 Correct reference to the redox reaction with KMnO4 (provided that FA 8 was identified as an alcohol/aldehyde). 1 3(b)(iii) FA 7 is an oxidising agent because iodine is formed. 1

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