Cambridge A Level Chemistry 9701 — 2017 Oct/Nov Paper 3 · Variant 3

1 In this experiment you will determine the value of x in the formula for hydrated copper(II) sulfate, CuSO4.xH2O. You will first react a solution of Cu2+ ions with excess iodide ions, I–. This reaction produces iodine. 2Cu2+(aq) + 4I–(aq) 2CuI(s) + I2(aq) The amount of iodine produced can be determined by titrating with thiosulfate ions, S2O32–. I2(aq) + 2S2O32–(aq) 2I–(aq) + S4O62–(aq) FA 1 is 0.150 mol dm–3 sodium thiosulfate, Na2S2O3. FA 2 is dilute sulfuric acid. FA 3 is 1.00 mol dm–3 potassium iodide, KI. FA 4 is a solution made by dissolving 32.5 g of CuSO4.xH2O in 1.00 dm3 of solution. starch indicator (a) Method ● Fill the burette with FA 1. ● Pipette 25.0 cm3 of FA 4 into a conical flask. ● Use the measuring cylinder to add 10 cm3 of FA 2 to the same conical flask. ● Use the measuring cylinder to add 10 cm3 of FA 3 to the same conical flask. The mixture will become brown because of the formation of I2, and will become cloudy because of the formation of the white precipitate of CuI. ● Add FA 1 from the burette until the mixture becomes a light brown colour. ● Add 10 to 20 drops of starch indicator until the mixture becomes blue-black. ● Continue to titrate with FA 1 until the blue-black colour disappears leaving a mixture with an off-white solid. This is the end-point. ● You should test that the end-point has been reached by adding 2 more drops of starch indicator. If the titration has reached the end-point the added starch indicator will cause no change in colour. ● 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 1 added in each accurate titration. I II III IV V VI VII [7] (b) From your accurate titration results, obtain a suitable value for the volume of FA 1 to be used in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FA 4 required ............................. cm3 of FA 1. [1] (c) Calculations Show your working and appropriate significant figures in the final answer to each step of your calculations. (i) Calculate the number of moles of thiosulfate ions in the volume of FA 1 calculated in (b). moles of S2O32– = ............................. mol (ii) Using the equations on page 2, calculate the number of moles of copper(II) ions in 25.0 cm3 of FA 4. moles of Cu2+ = ............................. mol (iii) Calculate the concentration, in mol dm–3, of copper(II) ions in FA 4. concentration of Cu2+ in FA 4 = ............................. mol dm–3 (iv) Calculate the value of x in CuSO4.xH2O. x = ……….........…….. [5] (d) (i) Calculate the maximum percentage error in one of your accurate titres. maximum percentage error = ............................. % (ii) A student suggests that the experiment could be made more accurate if the volume of FA 3 was measured using a burette. Give a reason why the student might make this suggestion. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. Explain why this change would not improve the accuracy of the experiment. ............................................................................................................................................. ............................................................................................................................................. [3] [Total: 16] Question 2 starts on the next page.

Mark scheme: 1(a) I The following data is shown • two burette readings for the rough titration • titre for rough titration • initial and final burette readings for two (or more) accurate titrations (i.e. 2 × 2 “box”) 1 II Appropriate headings and units for accurate titration. and volume FA 1 added recorded for each accurate titre. Headings should match readings. • initial / start and (burette) reading / volume (allow vol but not V) • final / end and (burette) reading / volume • titre or volume / FA 1 and used/added (but not “difference” or “total” or “change”) unit: / cm3 or (cm3) or in cm3 or cm3 for each entry 1 III All accurate burette readings are to the nearest 0.05 cm3. The requirement to record to 0.05 applies to burette readings, including 0.00 cm3 (if this was the initial reading), but it does not apply to the titre. Do not award this mark if: • 50(.00) is used as an initial burette reading • more than one final burette reading is 50.(00) • any burette reading is greater than 50.(00) 1 IV The final accurate titre recorded is within 0.10 cm3 of any other accurate titre. 1 Examiner rounds any accurate burette readings to the nearest 0.05 cm3 and then selects the ‘best’ titres using the hierarchy: • two (or more) accurate identical titres, then • two (or more) accurate titres within 0.05 cm3, then • two (or more) accurate titres within 0.10 cm3 etc. These best titres should be used to calculate the mean corrected titre to the nearest 0.01 cm3. Examiner compares candidate’s titre value with that of the Supervisor: Question Answer Marks 1(a) Award V, VI and VII if δ ⩽ 0.30 (cm3) 1 Award V and VI if 0.30 < δ ⩽ 0.60 1 Award V, only, if 0.60 < δ ⩽ 1.00 1 1(b) Candidate calculates the mean correctly. • Candidate averages two (or more) titres where the total spread is ⩽ 0.20 cm3. • Working 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. (e.g. 26.666 cm3 must be rounded to 26.67 cm3) Two special cases, where the mean need not be to 2 dp: • Allow mean to 3 dp only for 0.025 or 0.075 (e.g. 26.325 cm3) • Allow mean to 1 dp, if all accurate burette readings were given to 1 dp and the mean is exactly correct. (e.g. 26.0 and 26.2 = 26.1 is allowed) (e.g. 26.0 and 26.1 = 26.1 is wrong – should be 26.05) Do not award this mark if: • The rough titre was used to calculate the mean. • The candidate performed only one accurate titration. • Burette readings were incorrectly subtracted to obtain any of the accurate titre values. • All burette readings (resulting in titre values used in calculation of mean) are integers. Note: the candidate’s mean will sometimes be marked correct even if it was different from the mean calculated by the Examiner for the purpose of assessing accuracy. 1 1(c)(i) Correctly calculates Number of moles of S2O3 2– used = 0.150 × 1000 (b) Answer given to 3 or 4 sf 1 1(c)(ii) Correctly calculates ans(ii) = ans(i) Answer given to 3 or 4 sf 1 Question Answer Marks 1(c)(iii) Correct use ans(ii) / 0.0250 (or equivalent) Answer given to 3 or 4 sf 1 1(c)(iv) Correct expression 32.5 / ans(iii) – 159.6 1 Correct answer x = nearest integer to [32.5 / ans 159.6] 18 − (iii) 1 1(d)(i) Correct expression Use of 0.1(0) 100 any accurate titre × 1 1(d)(ii) The volume from the burette has a smaller error / more precise 1 FA 3 is in excess 1

More questions on Reacting masses and volumes (of solutions and gases)

2 In this experiment you will determine the value of y in the formula for hydrated barium chloride, BaCl 2.yH2O. You will do this by measuring the mass loss when a sample of hydrated barium chloride is heated. BaCl 2.yH2O(s) BaCl 2(s) + yH2O(g) FA 5 is hydrated barium chloride, BaCl 2.yH2O. (a) Method Before starting any practical work, read through all the instructions and prepare a table for your results in the space provided. ● Weigh the crucible with a lid and record the mass. ● Add all the FA 5 to the crucible. ● Reweigh the crucible with the lid and FA 5. Record the mass. ● Place the crucible in the pipe-clay triangle on top of a tripod. ● Heat the crucible gently with the lid on for about 1 minute. ● Remove the lid and then heat more strongly for a further 4 minutes. ● Replace the lid and allow the crucible to cool. ● While the crucible is cooling you may wish to begin work on Question 3. ● Once the crucible has cooled, reweigh the crucible with the lid and contents. Record the mass. ● Calculate and record the mass of FA 5 used, the mass of the residue and the mass of water lost. I II III IV V [5] (b) Calculations Show your working and appropriate significant figures in the final answer to each step of your calculations. (i) Calculate the number of moles of barium chloride in the residue. You may assume all the water has been removed. moles of BaCl 2 = ............................. mol (ii) Calculate the number of moles of water lost. moles of H2O lost = ............................. mol (iii) Calculate the value of y in BaCl 2.yH2O. y = ............................. [3] (c) (i) For this experiment to give an accurate value for y, anhydrous barium chloride must be thermally stable. Explain fully what would happen to the value of y if BaCl 2 were to decompose slightly during heating. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (ii) Starting with the same mass of hydrated barium chloride, suggest how you could modify the experiment to determine more accurately the mass of water lost. ............................................................................................................................................. ............................................................................................................................................. [3] [Total: 11]

Mark scheme: 2(a) I Table of data Must show all of the following: • mass of crucible (+ lid) • mass of crucible (+ lid) + FA 5 • mass of crucible (+ lid) + residue • mass of FA 5 • mass of residue • mass of water lost 1 II Recording of data • Unit / g, (g) or in grams for all data recorded • all three balance readings recorded to same number of dp 1 III Correctly calculates • mass of FA 5, • mass of residue, • mass of water lost 1 Examiner checks supervisor’s subtraction for mass of FA 5 and mass of residue and calculates the ratio mass of FA 5 ÷ mass of residue to 2 dp. Examiner compares candidate’s value with that of Supervisor. Award IV if δ ⩽ 0.10 1 Award V if δ ⩽ 0.05 1 2(b)(i) Correctly uses (i) = mass of residue / 208.3 Answer given to 2–4 sf 1 2(b)(ii) Correctly calculates (ii) = mass of water lost / 18 Answer given to 2–4 sf 1 2(b)(iii) Correctly calculates (ii) ÷ (i) and y as an integer 1 Question Answer Marks 2(c)(i) Greater mass lost / smaller mass of residue / fewer moles of residue / greater mass of water (appears to be lost) 1 so y would be greater 1 2(c)(ii) heat to constant mass OWTTE / cooling in a desiccator 1

More questions on Reacting masses and volumes (of solutions and gases)

3 Qualitative Analysis At each stage of any test you are to record details of the following: ● colour changes seen; ● the formation of any precipitate; ● the solubility of such precipitates in an excess of the reagent added. Where reagents are selected for use in a test, the name or correct formula of the element or compound must be given. Where gases are released they should be identified by a test, described in the appropriate place in your observations. You should indicate clearly at what stage in a test a change occurs. No additional tests for ions present should be attempted. If any solution is warmed, a boiling tube MUST be used. Rinse and reuse test-tubes and boiling tubes where possible. FA 6 and FA 7 are both salts which contain cations and anions from those listed in the Qualitative Analysis Notes. Each salt contains a single cation and a single anion. (a) Carry out the following test and record your observations. observations test FA 6 FA 7 (i) Place a small spatula measure of the solid in a hard-glass test-tube and heat gently at first, then more strongly. (ii) From your observations, what is present in both salts? ............................................................................................................................................. [4] (b) Prepare solutions of FA 6 and FA 7 by placing the rest of each solid into separate 100 cm3 beakers. Add approximately 30 cm3 of distilled water to each beaker and stir until fully dissolved. Use these solutions for tests in (b). (i) Carry out tests to determine the cation present in each solution. Record your tests and results in the space below. (ii) Carry out all the following tests and record your observations. observations test solution of FA 6 solution of FA 7 To a 1 cm depth of solution in a test-tube add a 1 cm depth of barium chloride or barium nitrate, then add an excess of hydrochloric acid or nitric acid. To a 1 cm depth of solution in a test-tube add a 1 cm depth of silver nitrate. (iii) Identify the ions present in each salt. FA 6 contains ..................................................... and ..................................................... . FA 7 contains ..................................................... and ..................................................... . [9] [Total: 13] Qualitative Analysis Notes 1 Reactions of aqueous cations reaction with ion NaOH(aq) NH3(aq) aluminium, white ppt. white ppt. Al 3+(aq) soluble in excess insoluble in excess ammonium, no ppt. – NH4+(aq) ammonia produced on heating barium, faint white ppt. is nearly always no ppt. Ba2+(aq) observed unless reagents are pure calcium, white ppt. with high [Ca2+(aq)] no ppt. Ca2+(aq) chromium(III), grey-green ppt. grey-green ppt. Cr3+(aq) soluble in excess insoluble in excess copper(II), pale blue ppt. blue ppt. soluble in excess Cu2+(aq) insoluble in excess giving dark blue solution green ppt. turning brown on contact green ppt. turning brown on contact iron(II), with air with air Fe2+(aq) insoluble in excess insoluble in excess iron(III), red-brown ppt. red-brown ppt. Fe3+(aq) insoluble in excess insoluble in excess magnesium, white ppt. white ppt. Mg2+(aq) insoluble in excess insoluble in excess off-white ppt. rapidly turning brown off-white ppt. rapidly turning brown manganese(II), on contact with air on contact with air Mn2+(aq) insoluble in excess insoluble in excess zinc, white ppt. white ppt. Zn2+(aq) soluble in excess soluble in excess

Mark scheme: 3(a)(i) FA 6 (Heating) produces water vapour / steam / moisture or condensation / solution / liquid forms / melts / dissolves AND FA 7 (Heating) produces water vapour / steam / moisture or condensation / solution / liquid forms / melts 1 FA 6 (stronger heating) gives a white solid/ residue AND FA 7 a yellow / green / brown / black solid/ residue 1 Gas / chlorine / Cl2 from heating FA 7 bleaches damp litmus paper or Gas / hydrogen chloride / HCl from heating FA 7 turns litmus red. 1 3(a)(ii) water 1 Question Answer Marks 3(b)(i) Clear presentation of results to show FA 6 and FA 7 and two or more reagents. 1 Uses NaOH(aq) and NH3(aq). 1 FA 6 FA 7 NaOH white ppt and (pale / light) blue ppt and no change / insoluble with excess no change / insoluble with excess NH3 white ppt and (pale) blue ppt and no change / insoluble with excess dark / deep blue solution with excess Two boxes correct for each mark. 2 3(b)(ii) test observations + Ba2+(aq) FA 6 FA 7 white ppt no reaction / no ppt / no change + excess of HCl or HNO3 insoluble no reaction / no ppt / no change + Ag+(aq) no reaction / no ppt / no change white ppt Two boxes correct for each mark. 3 3(b)(iii) FA 6 contains Mg2+ / magnesium and SO4 2– / sulfate FA 7 contains Cu2+ / copper(II) and Cl – / chloride 1 mark for 2 correct ions 2

More questions on Some reactions of the halide ions