Cambridge A Level Chemistry 9701 — 2022 May/June Paper 3 · Variant 2

1 A bottle containing the acid salt sodium hydrogen sulfate, NaHSO4, has been contaminated. You will determine the percentage purity by mass of the sodium hydrogen sulfate by titrating a solution of the acid salt against a known concentration of sodium hydroxide. NaHSO4(aq) + NaOH(aq) → Na2SO4(aq) + H2O(l) The impurity in the sodium hydrogen sulfate does not react with aqueous sodium hydroxide under the conditions of the titration. FB 1 is 0.100 mol dm–3 sodium hydroxide, NaOH. FB 2 is 12.53 g dm–3 impure sodium hydrogen sulfate. FB 3 is thymol blue indicator. (a) Method ● Fill a burette with FB 1. ● Pipette 25.0 cm3 of FB 2 into a conical flask. ● Add approximately 10 drops of FB 3. ● Perform a rough titration and record your burette readings in the space below. The end‑point is shown by the appearance of a permanent blue colour. The rough titre is .............................. cm3. ● Carry out as many titrations as you think necessary to obtain consistent results. ● Make certain any recorded results show the precision of your practical work. ● Record, in a suitable form below, all your burette readings and the volume of FB 1 added in each accurate titration. I II III IV V VI VII [7] (b) From your accurate titration results, calculate a suitable mean value to use in your calculations. Show clearly how you obtain the mean value. 25.0 cm3 of FB 2 required .............................. cm3 of FB 1. [1] (c) Calculations (i) Give all your answers to (c)(ii), (c)(iii) and (c)(iv) to an appropriate number of significant figures. [1] (ii) Use your answer to (b) to calculate the amount, in mol, of sodium hydroxide, FB 1, titrated. amount of NaOH = .............................. mol Hence, deduce the amount, in mol, of sodium hydrogen sulfate present in 25.0 cm3 of FB 2. amount of NaHSO4 = .............................. mol [1] (iii) Use your final answer to (c)(ii) to calculate the mass of sodium hydrogen sulfate present in 1.00 dm3 of FB 2. mass of NaHSO4 = .............................. g [1] (iv) Use your answer to (c)(iii) and the information on page 2 to calculate the percentage purity by mass of the sodium hydrogen sulfate. percentage purity = .............................. % [1] [Total: 12]

Mark scheme: 1(a) I All the following data are recorded:  two burette readings and titre for the rough titration  initial and final burette readings for two (or more) accurate titrations 7 II Correct headings and units in the accurate titration table and titre values recorded for accurate titrations  initial / start and (burette) reading / volume + unit  final / end and (burette) reading / volume + unit  titre + unit or volume / FB 1 and used / added + unit III All accurate burette readings are recorded to the nearest 0.05 cm3 IV The final accurate titre recorded is within 0.10 cm3 of any other accurate titre For assessment of accuracy marks, round all burette readings to the nearest 0.05 cm3. Check and correct subtractions. Then select the ‘best’ titres using the hierarchy:  two (or more) accurate identical titres (ignoring any that are labelled ‘rough’), 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 titre, expressed to nearest 0.01 cm3. Write the candidate’s mean titre on each script. Write the Supervisor’s [corrected] mean titre in a ring on each candidate script. Calculate the difference () between the candidate’s mean titre and the supervisor’s mean titre. Write the value of  on each script. Award the accuracy marks as shown below. 1(a) Award V if  ⩽ 0.50 (cm3) Award VI if  ⩽ 0.30 Award VII if  ⩽ 0.20 1(b) Correctly calculates the mean titre correct to 2 dp from accurate titres that are within 0.20 cm3 total spread 1 1(c)(i) All final answers for (c)(ii), (c)(iii), and (c)(iv) are to 3–4 sf 1 Question Answer Marks 1(c)(ii) Correctly calculates n(NaOH) = 0.1  (b) / 1000 AND n(NaHSO4) = same answer 1 1(c)(iii) Correctly uses [NaHSO4] = final answer (c)(ii)  1000 / 25 (= (c)(ii)  40) AND answer above  120.1 1 1(c)(iv) Correctly uses % NaHSO4 = final answer 12.53       (c)(ii)  100 1

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2 In Question 1 you carried out a neutralisation reaction involving sodium hydroxide. H+(aq) + OH–(aq) → H2O(l) In Question 2 you are to determine the enthalpy of neutralisation, ∆Hneut, as shown by the equation above. You will use a solution of sodium hydroxide and the diprotic acid, sulfuric acid. FB 4 is approximately 2 mol dm–3 sodium hydroxide, NaOH. FB 5 is 1.00 mol dm–3 sulfuric acid, H2SO4. (a) Method ● Place the cup in the 250 cm3 beaker. ● Use the 25.0 cm3 measuring cylinder to transfer 25.0 cm3 of FB 4 into the cup. ● Place the thermometer in the solution. Record the temperature. ● Fill the clean burette with FB 5. ● Run 5.00 cm3 of FB 5 into the same cup. ● Stir the mixture and record the highest temperature observed. ● Repeat adding 5.00 cm3 volumes of FB 5 into the same cup until 45.00 cm3 has been added. Record the highest temperature after each addition. Results Table 2.1 volume of FB 5 / cm3 0.00 5.00 10.00 15.00 20.00 temperature / °C volume of FB 5 / cm3 25.00 30.00 35.00 40.00 45.00 temperature / °C [3] (b) (i) Plot a graph of temperature (y-axis) against volume of acid added (x-axis) on the grid provided. Select a scale on the y-axis to include a temperature 2.0 °C above the highest temperature you recorded. Label any points you consider to be anomalous. Draw two lines of best fit, one for the rise in temperature and one for the temperature change after the maximum temperature has been reached. Extrapolate the two lines so they intersect. [4] (ii) Use your graph to: ● determine the volume of sulfuric acid, FB 5, required to neutralise 25.0 cm3 of sodium hydroxide, FB 4 ● determine the maximum change in temperature, ∆T. volume of H2SO4 = .............................. cm3 maximum ∆T = .............................. °C [1] (c) (i) Use your answer to (b)(ii) to calculate the amount, in mol, of sulfuric acid, FB 5, neutralised in your reaction. amount of H2SO4 = .............................. mol [1] (ii) Calculate the heat energy evolved in the neutralisation reaction in (a). (Assume that 4.18 J are required to change the temperature of 1.0 cm3 of solution by 1.0 °C.) heat energy evolved = .............................. J [1] (iii) Use your answers to (c)(i) and (c)(ii) to calculate the enthalpy of neutralisation, ∆Hneut, for the reaction given in (a). H+(aq) + OH–(aq) → H2O(l) ∆Hneut = ...... ............................. ..................... [1] sign value unit (d) (i) The value for quoted in a textbook is –57.6 kJ mol–1. Calculate the percentage error in your answer to (c)(iii) compared with the theoretical value. (If you were unable to answer (c)(iii) then assume the value was –49.2 kJ mol–1.) percentage error = .............................. % [1] (ii) Without changing the apparatus, suggest one improvement that could be made to the method in (a). Explain your answer. improvement ....................................................................................................................... ............................................................................................................................................. explanation .......................................................................................................................... ............................................................................................................................................. [1] (e) (i) Use your graph and the information on page 4 to calculate the concentration, in mol dm–3, of sodium hydroxide in FB 4. concentration of NaOH = .............................. mol dm–3 [1] (ii) A student repeats Question 1 with a new solution of FB 1. The student decides to dilute FB 4 by a factor of 20 to make the new FB 1 solution. The student incorrectly assumes the concentration of the new FB 1 solution is 0.100 mol dm–3. Calculate the actual concentration of NaOH in the new FB 1 solution. (If you were unable to answer (e)(i) then assume the concentration of sodium hydroxide in FB 4 was 1.93 mol dm–3. This is not the correct value.) concentration of NaOH in the new FB 1 solution = .............................. mol dm–3 Predict the effect that using the new FB 1 solution has on the value for the percentage purity of the sodium hydrogen sulfate you calculated in 1(c)(iv). The percentage purity of NaHSO4 would be larger than calculated. The percentage purity of NaHSO4 would be the same as calculated. The percentage purity of NaHSO4 would be smaller than calculated. Tick the appropriate box and explain your answer. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] [Total: 16] Qualitative analysis For each test you should record all your observations in the spaces provided. Examples of observations include: ● colour changes seen ● the formation of any precipitate and its solubility (where appropriate) in an excess of the reagent added ● the formation of any gas and its identification (where appropriate) by a suitable test. You should record clearly at what stage in a test an observation is made. Where no change is observed you should write ‘no change’. Where reagents are selected for use in a test, the name or correct formula of the element or compound must be given. If any solution is warmed, a boiling tube must be used. Rinse and reuse test-tubes and boiling tubes where possible. No additional tests should be attempted.

Mark scheme: 2(a) I Ten thermometer readings recorded, all shown to .0 or .5 ºC AND at least one reading at .0 and one at .5 3 For assessment of accuracy marks, round all thermometer readings to the nearest 0.5 °C. Calculate the supervisor’s greatest T (= Tmax – Tinitial) and write it ringed on each candidate script. Note volume (V) at which the Tmax occurs. Calculate the candidate’s T at the same volume (= Tat V – Tinital) then calculate the difference, , from the supervisor value. Award of accuracy marks For supervisor T 10.0 – 20.0: II Award if ⩽2.0 ºC III Award if ⩽1.0 ºC 2(b)(i) I Axes unambiguously labelled temperature or °C on y-axis AND volume / FB 5 / H2SO4 / acid or cm3 on the x-axis AND some numbers for scales 4 II Suitable scales chosen III All points recorded in the table are accurately plotted IV Lines of best fit drawn (straight or smoothly curved lines) AND extrapolated to intersect 2(b)(ii) Correct volume from suitable intersect to 1 or 2 dp AND correct T to 1 dp 1 2(c)(i) Correctly calculates n(H2SO4) = 1.00  volume (b)(ii) / 1000 AND answer to 2–4 significant figures 1 Question Answer Marks 2(c)(ii) Correctly calculates Q = (25 + V(b)(ii))  4.18  T(b)(ii) and answer to 2–4 sf 1 2(c)(iii) Correctly uses H = 2        (c)(ii) (c)(i) / (1000) AND answer given to minimum 2 sf AND units kJ mol−1 or J mol−1 to match use or absence of ÷ 1000 AND negative sign 1 2(d)(i) Correct expression [(− 57.6 – (c)(iii)) / 57.6]  100 OR [((c)(iii) – (– 57.6)) / 57.6]  100 1 2(d)(ii) take more readings (near the end point) AND to improve position of intersection / owtte 1 2(e)(i) Correctly uses n(NaOH) = (c)(i)  2  1000 25 1 Question Answer Marks 2(e)(ii) I Correctly uses 20 (e)(i) AND answer to 2 or more sf 2 II Explanation matches answer If conc NaOH > 0.1 mol dm−3 then 1st box ticked and (titre remains unchanged so actual) mol NaOH↑ so mol / amount / mass NaHSO4↑ OR If conc NaOH approx. 0.1 mol dm−3 then 2nd box ticked and titre not / (only) slightly changed so within experimental error OR If conc NaOH < 0.1 mol dm−3 then 3rd box ticked and (titre remains unchanged so actual) mol NaOH↓ so mol / amount / mass NaHSO4↓ OR If conc NaOH < 0.1 mol dm−3 then 1st box ticked and larger titre so moles / amount / mass NaHSO4 calculated increases OR If conc NaOH > 0.1 mol dm−3 then 3rd box ticked and smaller titre so mol / amount / mass NaHSO4 calculated decreases

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3 (a) FB 6 is the solid impurity found in the bottle of solid used to prepare FB 2. It is a compound of a Group 1 metal and does not contain sulfur. The anion in FB 6 is listed in the Qualitative analysis notes. Select a reagent, or reagents, and carry out one test on FB 6 to collect more information about the anion present. reagent or reagents .................................................................................................................... apparatus and conditions ........................................................................................................... .................................................................................................................................................... observations ............................................................................................................................... .................................................................................................................................................... From your observations give the formula of the anion present. If you are unable to identify the anion positively from your test and observations, then write ‘unknown’. formula of anion ............... [4] (b) (i) FB 7 and FB 8 are solutions containing a total of three cations. All of the cations are listed in the Qualitative analysis notes. Carry out the following tests and record your observations. Use a fresh 1 cm depth of solution in a test‑tube for each test. Table 3.1 observations test FB 7 FB 8 Test 1 Add a few drops of acidified aqueous potassium manganate(VII). Test 2 Add aqueous ammonia. Test 3 Add aqueous sodium hydroxide, then decant the mixture into a boiling tube and warm gently. [4] (ii) Using your observations in (b)(i), identify the cations present in FB 7 and FB 8. Write the formula of each cation identified. If the tests do not allow you to positively identify the cations, write ‘unknown’. cation or cations in FB 7 .................................................. cation or cations in FB 8 .................................................. [2] (iii) Construct the ionic equation for one reaction observed on addition of aqueous ammonia in Test 2. Include state symbols. ....................................................................................................................................... [1] (iv) Deduce the type of reaction which occurs when acidified aqueous potassium manganate(VII) is added to FB 7 in Test 1 in (b)(i). ....................................................................................................................................... [1] [Total: 12] Qualitative analysis notes 1 Reactions of cations cation reaction with NaOH(aq) NH3(aq) aluminium, Al 3+(aq) white ppt. soluble in excess white ppt. insoluble in excess ammonium, NH4+(aq) no ppt. – ammonia produced on warming barium, Ba2+(aq) faint white ppt. is observed unless no ppt. [Ba2+(aq)] is very low calcium, Ca2+(aq) white ppt. unless [Ca2+(aq)] is very no ppt. low chromium(III), Cr3+(aq) grey-green ppt. soluble in excess grey-green ppt. insoluble in excess giving dark green solution copper(II), Cu2+(aq) pale blue ppt. insoluble in excess pale blue ppt. soluble in excess giving dark blue solution iron(II), Fe2+(aq) green ppt. turning brown on green ppt. turning brown on contact with air contact with air insoluble in excess insoluble in excess iron(III), Fe3+(aq) red-brown ppt. insoluble in excess red-brown ppt. insoluble in excess magnesium, Mg2+(aq) white ppt. insoluble in excess white ppt. insoluble in excess manganese(II), Mn2+(aq) off-white ppt. rapidly turning brown off-white ppt. rapidly turning brown on contact with air on contact with air insoluble in excess insoluble in excess zinc, Zn2+(aq) white ppt. soluble in excess white ppt. soluble in excess 2 Reactions of anions anion reaction carbonate, CO32– CO2 liberated by dilute acids chloride, Cl –(aq) gives white ppt. with Ag+(aq) (soluble in NH3(aq)) bromide, Br –(aq) gives cream / off-white ppt. with Ag+(aq) (partially soluble in NH3(aq)) iodide, I–(aq) gives pale yellow ppt. with Ag+(aq) (insoluble in NH3(aq)) nitrate, NO3–(aq) NH3 liberated on heating with OH–(aq) and Al foil nitrite, NO2–(aq) NH3 liberated on heating with OH–(aq) and Al foil; decolourises acidified aqueous KMnO4 sulfate, SO42–(aq) gives white ppt. with Ba2+(aq) (insoluble in excess dilute strong acids); gives white ppt. with high [Ca2+(aq)] sulfite, SO32–(aq) gives white ppt. with Ba2+(aq) (soluble in excess dilute strong acids); decolourises acidified aqueous KMnO4 thiosulfate, S2O32–(aq) gives off-white / pale yellow ppt. slowly with H+

Mark scheme: 3(a) I Suitable reagents for anion One of: If testing for carbonate: I named (dilute) acid (and limewater) II description of testing gas with limewater III no effervescence / no fizzing / no change IV unknown (provided test carried out and no contradictory result) OR If testing for halide: I (aqueous) AgNO3 and NH3 II use a solution of FB 6 III (pale) yellow ppt, insoluble in NH3 IV I– (must be from (pale) yellow ppt) OR If testing for nitrate / nitrite : I sodium hydroxide and Al II warm in a boiling tube III gas does not turn litmus blue / red litmus stays red IV unknown (provided test carried out and no contradictory result) Question Answer Marks 3(b)(i) FB 7 FB 8 + H+/ MnO4− turns brown OR brown solution OR (dark) brown ppt * purple (solution) / KMnO4 / MnO4– AND decolourised / turns yellow / turns colourless * + NH3 off-white / pale brown ppt AND insoluble in excess OR darkens / turns dark(er) brown * green ppt AND insoluble in excess OR turns brown / red-brown / rust (on standing) * + NaOH off-white / pale brown ppt AND insoluble in excess OR darkens / turns dark(er) brown * green ppt AND insoluble in excess OR turns brown / red-brown / rust (on standing) * & warm gas / NH3 turns (damp red) litmus blue * no (visible) reaction / no change * 2 * = 1 mark 4 3(b)(ii) FB 7: Mn2+ and NH4+ FB 8: Fe2+ 3 cations correct = 2 marks 2 cations correct = 1 mark 2 3(b)(iii) One of: Mn2+(aq) + 2OH−(aq) → Mn(OH)2(s) Fe2+(aq) + 2OH−(aq) → Fe(OH)2(s) 1 3(b)(iv) Redox / oxidation AND reduction (of manganate(VII)) 1

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