Cambridge A Level Chemistry 9701 — 2017 May/June Paper 3 · Variant 3

1 Sodium hydrogencarbonate, NaHCO3, is used as baking soda in cooking. Baking soda may also contain small amounts of other chemicals. In this experiment, you will determine the percentage purity by mass of an impure sample of NaHCO3 by titration with sulfuric acid. FA 1 is 0.0500 mol dm–3 sulfuric acid, H2SO4. FA 2 is impure NaHCO3. methyl orange (a) Method Preparing a solution of FA 2 ● Weigh the stoppered container of FA 2. Record the mass in the space below. ● Tip all the FA 2 into the beaker. ● Reweigh the container with its stopper. Record the mass. ● Calculate and record the mass of FA 2 used. ● Add approximately 100 cm3 of distilled water to the FA 2 in the beaker. ● Stir the mixture with a glass rod until all the FA 2 has dissolved. ● Transfer this solution into the 250 cm3 volumetric flask. ● Wash the beaker with distilled water and transfer the washings to the volumetric flask. ● Rinse the 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 impure NaHCO3 is FA 3. Label the flask FA 3. Results Titration ● Fill the burette with FA 1. ● Pipette 25.0 cm3 of FA 3 into a conical flask. ● Add several drops of methyl orange. ● 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 Keep FA 1 for use in Question 2. II III IV V VI VII VIII [8] (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 3 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 sulfuric acid present in the volume of FA 1 calculated in (b). moles of H2SO4 = ............................. mol (ii) Balance the equation for the reaction of sulfuric acid and sodium hydrogencarbonate. State symbols are not required. .......NaHCO3 + .......H2SO4 .......Na2SO4 + .......CO2 + .......H2O (iii) Using your answers to (i) and (ii), calculate the number of moles of sodium hydrogencarbonate used in each titration. moles of NaHCO3 = ............................. mol (iv) Using your answer to (iii), calculate the mass of sodium hydrogencarbonate present in the mass of FA 2 used to prepare FA 3. mass of NaHCO3 = ............................. g (v) Calculate the percentage purity by mass of the impure sodium hydrogencarbonate sample, FA 2. percentage purity by mass of impure NaHCO3, FA 2 = ............................. % (vi) What did you assume about the impurities in FA 2 when you calculated the percentage purity? ............................................................................................................................................. ............................................................................................................................................. (vii) A volumetric flask was labelled 250.0 ± 0.10 cm3. Calculate the maximum percentage error when using this volumetric flask. maximum percentage error = ............................. % [7] [Total: 16]

Mark scheme: 1(a) I Correct headings The following data are recorded in the space provided • mass of container with FA 2 • mass of (empty) container • mass of FA 2 ‘Mass’ must be stated for each piece of data. Unit / g (etc.) must be given for each piece of data. Subtraction for mass of FA 2 used must be correct. 1 II 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 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 / FA 1 and used / added • 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. 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. This mark is not awarded 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 V The final accurate titre recorded is within 0.10 cm3 of any other accurate titre. • Do not include a reading if it is labelled “rough”. • Do not award the mark if any ‘accurate’ burette readings (apart from initial 0 cm3) are given to zero dp. 1 Question Answer Marks For assessment of accuracy (Q) marks, each Examiner should round any burette readings to the nearest 0.05 cm3, check subtractions and 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. Calculate the candidate’s ratio to 1 dp, as shown below. Ratio = correct mean titre ÷ correct mass Calculate the difference (δ) between the candidate’s ratio and the supervisor’s ratio. Accuracy marks are awarded as follows. 1(a) Award VI, VII and VIII if δ ⩽ 0.2 (cm3 g–1) 1 Award VI and VII if 0.2 < δ ⩽ 0.4 1 Award VI, only, if 0.4 < δ ⩽ 0.6 1 • Spread penalty: if the two “best” (corrected) titres used by the Examiner were ⩾ 0.50 cm3 apart, maximum 2 accuracy marks. • If only a rough titration is shown, award Q marks based on that, maximum 2 accuracy marks. Question Answer Marks 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. (e.g. 26.665 cm3 must be rounded to 26.67 cm3) Two special cases, where the mean need not be to 2 dp: • Allow mean expressed to 3 dp only for 0.025 or 0.075 (e.g. 26.325 cm3) • Allow mean if expressed to 1 dp, if all accurate burette readings (apart from initial 0) 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) This mark is not awarded if: • The rough titre was used to calculate the mean. • The candidate did only one accurate titration. • Burette readings were incorrectly subtracted to obtain any of the accurate titre values. • All burette readings used to calculate the mean were recorded as 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) No of moles of H2SO4 used = 0.05(0) × (b)/ 1000 to minimum 2 sf 1 1(c)(ii) and 1(c)(iii) 2NaHCO3 + H2SO4 Æ Na2SO4 + 2CO2 + 2H2O and No of moles of NaHCO3 = 2 × answer (i) 1 Question Answer Marks 1(c)(iv) Mass of NaHCO3 = answer (iii) × 10 × 84 1 1(c)(v) % = answer (iv) / mass of FA 2 used × 100 1 All answers attempted in (i), (iii), (iv) & (v) are shown to 3 or 4 sf Minimum 3 answers attempted to gain the mark 1 1(c)(vi) Any one of the following answers. • the impurity does not react with (sulfuric) acid / FA 1 / NaHCO3 • the impurity is not alkaline / acidic • the impurity is neutral 1 1(c)(vii) % error (= 0.1 / 250 × 100) = 0.04% 1 Total: 16

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2 When baking soda is heated, carbon dioxide is produced. In this experiment you will investigate the reaction taking place when the sodium hydrogencarbonate in baking soda is thermally decomposed. FA 4 is baking soda (impure NaHCO3). Its composition is the same as that of FA 2. (a) Method Record all your readings in the space below. ● Weigh the crucible with its lid. ● Transfer all the FA 4 from the container into the crucible. ● Weigh the crucible, lid and FA 4. ● Calculate and record the mass of FA 4 used. ● Place the crucible and contents on a pipe-clay triangle. ● Heat gently, with the lid on, for approximately one minute. ● Heat strongly, with the lid off, for a further three minutes. ● Replace the lid and leave the crucible to cool for at least five minutes. While the crucible is cooling you may wish to begin work on Question 3. ● When it is cool, weigh the crucible with its lid and contents. ● Heat strongly, with the lid off, for a further two minutes. ● Replace the lid and leave the crucible to cool for at least five minutes. ● When it is cool, weigh the crucible with its lid and contents. ● Calculate and record the mass of residue obtained. ● This residue is FA 5. Keep this for use in 2(d). Results I II III IV [4] (b) Calculations Show your working and appropriate significant figures in the final answer to each step of your calculations. (i) Use the percentage purity by mass of FA 2 you calculated in 1(c)(v), to calculate the mass of sodium hydrogencarbonate in the sample of FA 4 that you weighed out. (If you were unable to carry out the calculation in 1(c)(v), assume that the percentage purity by mass of FA 2 is 95.8%.) mass of NaHCO3 in FA 4 weighed out = ............................. g (ii) Calculate the mass of impurity present in your sample of FA 4. mass of impurity = ............................. g (iii) The impurity in FA 4 does not decompose when it is heated. This means that the residue, FA 5, contains the mass of impurity calculated in (ii) together with the solid decomposition product of sodium hydrogencarbonate. Calculate the mass of the solid decomposition product. mass of solid decomposition product = ............................. g (iv) Use your answers to (i) and (iii) to calculate the mass of solid decomposition product that would be obtained if 84.0 g of pure sodium hydrogencarbonate were heated. mass of solid decomposition product = ............................. g (v) A student carried out the experiment by heating to constant mass and calculated that heating 84.0 g of pure NaHCO3 would produce 52.3 g of the solid decomposition product. The student then suggested the following equation for the thermal decomposition of sodium hydrogencarbonate. NaHCO3(s) NaOH(s) + CO2(g) Use data from the Periodic Table on page 12 to explain why the student’s suggestion cannot be correct. ............................................................................................................................................. ............................................................................................................................................. [4] (c) (i) Why was the lid put on while the crucible and its contents cooled? ............................................................................................................................................. ............................................................................................................................................. (ii) The experiment could be made more accurate by heating to constant mass or using a more accurate balance. Suggest a further improvement to make the experiment more accurate. ............................................................................................................................................. ............................................................................................................................................. [2] (d) (i) Pour a 1 cm depth of sulfuric acid, FA 1, into a test-tube. Add some FA 5 from the crucible to the acid in the test-tube. Record all your observations. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (ii) Use your observation(s) in (i) to identify an anion present in FA 5. Explain your answer. identity ........................................................... explanation .......................................................................................................................... ............................................................................................................................................. (iii) Steam is one of three products obtained when sodium hydrogencarbonate is thermally decomposed. Use your answer in (ii) to complete and balance the equation for the thermal decomposition of sodium hydrogencarbonate. Include state symbols. ........NaHCO3(s) ........H2O(g) + ........CO2(g) + ......................... (iv) State whether the balanced equation in (iii) agrees with the student’s results given in 2(b)(v). Show working in order to explain your answer. ............................................................................................................................................. ............................................................................................................................................. [4] [Total: 14]

Mark scheme: 2(a) I Four weighings recorded and correct headings given and mass of FA 4 used and mass of residue recorded • (Mass of) crucible, (lid) • (Mass of) crucible, (lid) and FA 4 (or 'contents before heating) • (Mass of) crucible, (lid) and contents / residue / FA 4 after (first) heating • (Mass of) crucible, (lid) and contents / residue / FA 4 after re-heating • (Mass of) FA 4 • (Mass of) residue / FA 5 / contents after heating If ‘mass’ not written then ‘g’ must be with each entry. Use of lid must be consistent. 1 II • All weighings recorded to same decimal places (one or more). • Third and fourth weighings are within 0.05 g of each other (or both equal if a one decimal place balance was used) • Mass of FA 4 and FA 5 / residue must be correctly subtracted. 1 Question Answer Marks 2(a) III and IV: • For assessment of accuracy, examiner must check and correct (if necessary) the masses of FA 4 used and of residue (smaller mass) obtained by the supervisor and by the candidate. • Work out ratio mass of FA4/mass of residue for the supervisor (2 dp) • Work out ratio mass of FA4/mass of residue for candidate (2 dp) • Calculate the difference (δ) between these two ratios. Award III and IV if δ ≤ 0.05 Award III if 0.05 < δ ≤ 0.10 2 2(b)(i) and 2(b)(ii) (i) Mass NaHCO3 = (% purity from 1(c)(v)/ 100) × mass of FA 4 used and (ii) Mass impurity = mass of FA 4 – answer (i) or mass impurity = % impurity / 100 x mass FA 4 1 2(b)(iii) Mass of decomposition solid = mass of residue (FA 5) from table – mass of impurity (ii) and expressed to 2, 3 or 4 sig fig or mass of decomposition solid = mass of NaHCO3 – mass lost on heating [(i) – (mass FA 4 – mass FA 5)] 1 2(b)(iv) Mass of residue obtained = answer (iii) × 84/ answer (i) 1 Question Answer Marks 2(b)(v) If correct, (84 g) NaHCO3 would give 40 g residue / NaOH (owtte) or mole ratio 1:1.3 (so not 1:1) or Answers could refer to mass / moles of CO2 1 2(c)(i) Lid reduces / stops absorption of water (vapour) by solid / residue / FA 5 while cooling 1 2(c)(ii) Repeat the experiment and ignore anomalous results / to obtain concordant / consistent results or cool in a desiccator or use larger mass of FA 4 / contents / solid 1 2(d)(i) Any two observations required • fizzing / effervescence / bubbling • gas turns limewater milky / chalky / cloudy white / white ppt • solid dissolves / colourless solution forms • rapid/brisk effervescence = 2 observations 1 2(d)(ii) FA 5 contains carbonate ion / CO3 2– and reference to fizzing (with acid) or to CO2 liberated (with acid) or positive limewater test or correct equation 1 2(d)(iii) 2NaHCO3(s) → H2O(g) + CO2(g) + Na2CO3(s) 1 2(d)(iv) (From equation) 84 g NaHCO3 should give 0.5 × 106 g residue (= 53 g) and gives a (sensible) comment based on student’s 52.3 g 1 Total: 14

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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. (a) (i) FA 6 and FA 7 are aqueous solutions. Each solution contains one cation and one anion from those listed in the Qualitative Analysis Notes. Use 1 cm depths of FA 6 or FA 7 in test-tubes for the following tests. Complete the table by recording your observations. observations test FA 6 FA 7 Add a few drops of aqueous barium chloride or aqueous barium nitrate, then add dilute nitric acid. Add a few drops of aqueous silver nitrate. Add a small spatula measure of sodium carbonate. Shake the mixture. (ii) From your observations, deduce which solution, FA 6 or FA 7, has the lower pH. Give your evidence. solution with lower pH ..................... evidence .............................................................................................................................. ............................................................................................................................................. [4] (b) Choose two reagents that would allow you to identify the cations in FA 6 and FA 7. reagents ............................................................. and .............................................................. Use these reagents to test solutions FA 6 and FA 7. Record all your observations in the space below. [4] (c) Deduce the chemical formulae of FA 6 and FA 7. FA 6 ............................................. FA 7 ............................................. [2] [Total: 10] 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 – no change / no reaction / no ppt / solution stays colourless with both • FA 7 – white precipitate with Ba2+ and • white ppt (remains) / insoluble / no reaction with HNO3 1 AgNO3 test: both observations correct • FA 6 – white precipitate • FA 7 – no change / no reaction / solution stays colourless / no ppt 1 Na2CO3 test: both observations correct • FA 6 – no reaction / solid does not dissolve / no effervescence • FA 7 – fizzing / bubbling / effervescence / or gas / CO2 turns limewater milky / chalky / cloudy white / (forms) white ppt 1 3(a)(ii) FA 7 has lower pH and gas / CO2 given off / it fizzes (more rapidly if fizzing with both) with sodium carbonate 1 Question Answer Marks 3(b) Reagents: NaOH and NH3 (names or correct formulae) 1 Observations – (3 × 1 mark) • FA 6 + NaOH : off-white / buff / beige / light brown ppt • FA 6 + NH3 : off-white / buff / beige / light brown ppt 1 • FA 6 : both ppts insoluble in excess and darken / turn brown with either 1 • FA 7 + NaOH : white ppt and soluble in excess • FA 7 + NH3 : white ppt and insoluble in excess 1 3(c) Conclusions (one mark for each). • FA 6 is MnCl 2 • FA 7 is Al 2(SO4)3 2 Total: 10

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