Cambridge A Level Chemistry 9701 — 2023 Feb/March Paper 3 · Variant 3

1 Some metal carbonates occur in a basic form which means that the metal hydroxide is also present. The formula of one form of basic zinc carbonate is ZnCO3•2Zn(OH)2•xH2O, where x is an integer. In this experiment you will carry out a thermal decomposition to find the relative formula mass, Mr, and the value of x for a sample of basic zinc carbonate. FA 1 is basic zinc carbonate, ZnCO3•2Zn(OH)2•xH2O. (a) Method ● Weigh the empty crucible with its lid. Record the mass. ● Transfer all the FA 1 from the container into the crucible. ● Weigh the crucible, lid and FA 1. Record the mass. ● Place the crucible and contents on a pipe-clay triangle. ● Heat the crucible gently, with the lid on, for approximately 1 minute. ● Heat strongly, with the lid off, for a further 4 minutes. ● Replace the lid and leave the crucible to cool for at least 5 minutes. While the crucible is cooling you may wish to begin work on Question 2 or 3. ● When the crucible is cool, weigh the crucible with its lid and contents. Record the mass. ● Place the crucible and contents on the pipe-clay triangle. Remove the lid. ● Heat the crucible strongly for a further 2 minutes. ● Replace the lid and leave the crucible to cool for at least 5 minutes. ● When the crucible is cool, reweigh the crucible with its lid and contents. Record the mass. ● Calculate and record the mass of FA 1 used. ● Calculate and record the mass of residue obtained. Results I II III IV V [5] (b) Calculations The equation for the thermal decomposition is shown. ZnCO3•2Zn(OH)2•xH2O(s) 3ZnO(s) + CO2(g) + (x + 2)H2O(g) (i) Calculate the amount, in mol, of zinc oxide, ZnO, formed after heating. amount of ZnO = .............................. mol Hence, calculate the amount, in mol, of basic zinc carbonate in your sample of FA 1. amount of ZnCO3•2Zn(OH)2•xH2O = .............................. mol [1] (ii) Use your answer to (b)(i) and your results in (a) to calculate the relative formula mass, Mr, of basic zinc carbonate, FA 1. Mr of ZnCO3•2Zn(OH)2•xH2O = .............................. [1] (iii) Use the Periodic Table to calculate the relative formula mass, Mr, of ZnCO3•2Zn(OH)2. Mr of ZnCO3•2Zn(OH)2 = .............................. [1] (iv) Use your answers to (b)(ii) and (b)(iii) to determine the value of x in ZnCO3•2Zn(OH)2•xH2O. Show your working. x = .............................. [2] [Total: 10]

Mark scheme: Question Answer Marks 1(a) I Table or list of correct headings for 4 balance readings: 1 • (mass of) crucible (and lid) • (mass of) crucible (and lid) and FA 1 / contents before heating • (mass of) crucible (and lid) and residue / contents after heating (not FA 1) • (mass of) crucible (and lid) and residue / contents after re-heating (not FA 1) II Recording of readings – all 3 bullets required: 1 • Unambiguous unit for all data is recorded • All weighings recorded to same number of decimal places (2 or more) • 4th weighing is ⩽ 0.05 g less than 3rd and is not more than 0.03 g greater than 3rd III Correct subtractions of masses 1 Masses of FA 1 and residue are correct. IV, V Accuracy (Q) marks • Examiner must check and correct (if necessary) the masses of FA 1 used and of residue obtained by supervisor and by candidate • Calculate (mass of FA 1) / (mass of residue) for the supervisor to 2 dp • Write this ratio, in a ring, on the candidate script • Calculate (mass of FA 1) / (mass of residue) for the candidate to 2 dp • Calculate δ, the difference between these two ratios Award IV if δ ⩽ 0.25 1 Award V if ⩽ 0.10 1 1(b)(i) Correctly calculates of moles of zinc oxide: 1 n(ZnO) = mass of residue / 81.4 and Correctly uses n(ZnO) to calculate moles of ZnCO3•2Zn(OH)2•xH2O: n(ZnO) / 3 and both answers given to 2–4 sf 1(b)(ii) Correctly uses (b)(i) to calculate Mr of ZnCO3•2Zn(OH)2•xH2O: 1 mass of FA 1 / final answer (b)(i) and answer given to 2–4 sf 1(b)(iii) Correctly calculates Mr as 324.2 1 1(b)(iv) M1 shows use of: 1 (b)(ii) – (b)(iii) / 18 M2 answer given correctly rounded to the nearest integer value 1

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2 In this experiment you will determine the relative formula mass, Mr, of basic zinc carbonate, ZnCO3•2Zn(OH)2•xH2O, by an alternative method. A known mass of basic zinc carbonate is reacted with a known volume and concentration of hydrochloric acid, HCl. The acid added is in excess. You will titrate portions of the resulting solution with sodium hydroxide, NaOH, of known concentration. FA 2 has been prepared as follows: 3.52 g of basic zinc carbonate, ZnCO3•2Zn(OH)2•xH2O, is reacted with 100 cm3 of 2.00 mol dm−3 hydrochloric acid, HCl. The resulting solution is diluted to 1.00 dm3 with distilled water. FA 3 is 0.150 mol dm−3 sodium hydroxide, NaOH. FA 4 is bromophenol blue indicator. (a) Method ● Fill the burette with FA 3. ● Pipette 25.0 cm3 of FA 2 into a conical flask. ● Add several drops of FA 4 to 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 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 your burette readings and the volume of FA 3 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 FA 2 required .............................. cm3 of FA 3. [1] (c) Calculations (i) Give your answers to (c)(ii), (c)(iii), (c)(iv) and (c)(vi) to an appropriate number of significant figures. [1] (ii) Calculate the amount, in mol, of sodium hydroxide present in the volume of FA 3 in (b). amount of NaOH = .............................. mol [1] (iii) Use your answer to (c)(ii) to calculate the amount, in mol, of hydrochloric acid present in 1.00 dm3 of FA 2. amount of HCl in 1.00 dm3 of FA 2 = .............................. mol [1] (iv) Use your answer to (c)(iii) and the information about the preparation of FA 2 to calculate the amount, in mol, of hydrochloric acid that reacted with 3.52 g of basic zinc carbonate. amount of HCl that reacted = .............................. mol [1] (v) Using the equation given in question 1(b) as a guide, complete the equation for the reaction of hydrochloric acid with basic zinc carbonate. State symbols are not required. ZnCO3•2Zn(OH)2•xH2O + 6HCl ................... + ................... + ................... [1] (vi) Use your answer to (c)(iv) and the equation in (c)(v) to calculate the amount, in mol, of basic zinc carbonate in 3.52 g. amount of ZnCO3•2Zn(OH)2•xH2O = .............................. mol Hence, calculate the relative formula mass, Mr, of basic zinc carbonate. Mr of ZnCO3•2Zn(OH)2•xH2O = .............................. [2] (d) A student correctly followed all the instructions in (a) to (c) of this question. The formula mass the student calculated was smaller than the correct value. The student suggests that this may be because the concentration of sodium hydroxide, FA 3, is greater than 0.150 mol dm–3. Explain why the student is correct. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [2] [Total: 17] 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 Initial and final readings and titre recorded for rough titration and accurate titration details tabulated 1 II Titre values recorded for accurate titrations 1 and All three headings and units correct for accurate titrations Headings: initial / final (burette) and reading / volume / vol or reading / volume / vol at start / finish and titre or volume / FA 3 and added / used and Units: (cm3) or / cm3 or in cm3 (or cm3 by every entry) III All accurate burette readings are recorded to the nearest 0.05 cm3 1 IV The final accurate titre recorded is within 0.10 cm3 of any other accurate titre 1 V, VI, VII 3 Award V if  ⩽ 0.50 cm3 Award VI if  ⩽ 0.30 cm3 Award VII if  ⩽ 0.20 cm3 2(b) Candidate must average two (or more) accurate titres with total spread of not more than 0.20 cm3. 1 The mean value must be quoted correct to 2 dp Working must be shown or ticks must be put next to the two (or more) 2(c)(i) All answers in (c)(ii), (c)(iii), (c)(iv) and (c)(vi) given to 3 or 4 sf 1 Minimum of three answers to qualify 2(c)(ii) Correctly calculates amount of NaOH added: 1 ((b) / 1000)  0.150 2(c)(iii) Correctly calculates amount of HCl in 1 dm3 of FA 2: 1 (c)(ii)  40 2(c)(iv) Correctly uses: 1 (c)(iv) = 0.2(0) – (c)(iii) 2(c)(v) Correctly balanced equation: 1 ZnCO3•2Zn(OH)2•xH2O + 6HCl → 3 ZnCl2 + CO2 + (x+5) H2O 2(c)(vi) Correctly uses: 1 (c)(iv) / 6 Correctly uses: 1 3.52 / ((c)(iv) / 6) 2(d) Any two for 1 mark and four for 2 marks: 2 • smaller titre / less FA 3 used / smaller volume (of NaOH) used • (therefore) calculated / remaining HCl is less or less HCl present in (1 dm3) FA 2 • more HCl apparently reacted (with basic zinc carbonate) • moles of basic zinc carbonate (reacting) increases • but same mass (so Mr is smaller)

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3 (a) FA 5 contains one cation and one anion, both of which are listed in the Qualitative analysis notes. FA 5 is insoluble in water. Transfer approximately half of the sample of FA 5 into a hard-glass test-tube. Heat the test- tube gently at first, then more strongly, until no further change occurs. Record all your observations. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [2] (b) FA 6 is an aqueous solution containing one cation and one anion. The anion is listed in the Qualitative analysis notes. FA 6 does not contain sulfur. (i) To a 2 cm depth of FA 6 in a test-tube, add a piece of magnesium ribbon. Record all your observations. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (ii) Write an ionic equation for the reaction between FA 6 and magnesium. Include state symbols. ..................................................................................................................................... [1] (c) (i) Add a small spatula measure of FA 5 to a 2 cm depth of FA 6 in a boiling tube. Record all your observations. The product of this test is FA 7. Label the boiling tube FA 7. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (ii) To a 1 cm depth of FA 7 in a test-tube, add aqueous ammonia. Record all your observations. ........................................................................................................................................... ..................................................................................................................................... [1] (d) (i) The same anion is present in both FA 6 and FA 7. The identity of this anion can be confirmed by testing either FA 6 or FA 7 with a pair of reagents. Select two pairs of reagents which may be used to identify the anion present in both FA 6 and FA 7. first pair of reagents: .............................................. and .............................................. second pair of reagents: .............................................. and .............................................. [2] (ii) Use FA 6 to carry out each test using the two pairs of reagents you have selected. Record all your observations in a suitable form below. [2] (e) Use your observations in (a) to (d) to identify the ions present in FA 5 and FA 6. If you are unable to identify an ion write ‘unknown’. cation anion FA 5 FA 6 [2] [Total: 13] 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 FA 5 is CuCO3 FA 6 is HCl 3(a) • Green powder 2 • (solid) turns black • (powder) ‘jumps’ around the tube • Condensation / (water) droplets • Gas / CO2 forms white ppt with limewater Any 2 bullet points = 1 mark 3(b)(i) • effervescence and Mg / solid dissolves / colourless solution formed / vigorous reaction / (test-tube / mixture) gets hot 1 • (gas) ‘pops’ with a lighted splint 1 3(b)(ii) Mg(s) + 2H+(aq) → Mg2+(aq) + H2(g) 1 3(c)(i) effervescence and blue / turquoise / green solution 1 3(c)(ii) (pale) blue ppt soluble in excess giving dark / deep blue solution 1 3(d)(i) silver nitrate and ammonia (solution) 1 sodium hydroxide and aluminium (foil) 1 3(d)(ii) M1 minimum 2  2 table with unambiguous heading(s) 1 and Use of both sets of reagents given in (d)(i) M2 With silver nitrate: white precipitate 1 and ppt soluble / colourless solution in ammonia and no reaction with second set of reagents 3(e) 2 cation anion FA 5 Cu2+ / copper(II) CO32– / carbonate FA 6 H+ / hydrogen Cl – / chloride Any 2 correct identities = 1 mark

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