Cambridge A Level Chemistry 9701 — 2018 May/June Paper 3 · Variant 5

1 In this experiment you will determine x in the formula of hydrated sodium carbonate, Na2CO3.xH2O, by titration. FA 1 is hydrated sodium carbonate, Na2CO3.xH2O. FA 2 is 0.110 mol dm–3 hydrochloric acid, HCl. methyl orange indicator (a) Method Making a solution of FA 1 ● Record all masses in the space below. ● Weigh the container with FA 1. ● Tip all the solid FA 1 into the 250 cm3 beaker. ● Weigh the container with any residual FA 1. ● Add approximately 100 cm3 of distilled water to the beaker and stir to dissolve FA 1. ● Transfer the solution to the 250 cm3 volumetric flask. ● Rinse the beaker twice, each time with about 20 cm3 of distilled water, and add this to the volumetric flask. ● Add distilled water to the volumetric flask to make 250 cm3 of solution and shake thoroughly. Label this solution FA 3. ● Calculate and record the mass of FA 1 used to make this solution. Titration ● Pipette 25.0 cm3 of FA 3 into a conical flask. ● Fill the burette with FA 2. ● Add several drops of methyl orange indicator to the conical flask. ● Carry out 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. I ● Make sure any recorded results show the accuracy of your practical work. ● Record, in a suitable form below, all of your burette readings and the volume of FA 2 II added in each accurate titration. III IV V VI VII [7] (b) From your accurate titration results, obtain a suitable value for the volume of FA 2 to be used in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FA 3 required .............................. cm3 of FA 2 [1] (c) Calculations (i) Give your answers to (ii), (iii) and (iv) to the appropriate number of significant figures. [1] (ii) Calculate the number of moles of hydrochloric acid in the volume of FA 2 calculated in (b). moles of HCl = .............................. mol [1] (iii) Complete the equation below and include the missing state symbols. Na2CO3(aq) + HCl (aq) NaCl + CO2 + H2O Calculate the number of moles of sodium carbonate in 25.0 cm3 of FA 3. moles of Na2CO3 in 25.0 cm3 of FA 3 = .............................. mol [1] (iv) Calculate the number of moles of sodium carbonate in 250.0 cm3 of FA 3. moles of Na2CO3 in 250.0 cm3 of FA 3 = .............................. mol Use this answer and your data on page 2 to calculate the relative formula mass, Mr, of hydrated sodium carbonate, Na2CO3.xH2O. Mr of Na2CO3.xH2O = .............................. [1] (v) Calculate the value of x in Na2CO3.xH2O. Give your answer to the nearest whole number. x = .............................. [1] (d) A student suggested using 0.110 mol dm–3 sulfuric acid in place of the 0.110 mol dm–3 hydrochloric acid used in the experiment above. The mass of FA 1 used was unchanged. Explain what effect this change would have on the accuracy of the experiment. .................................................................................................................................................... .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [2] [Total: 15]

Mark scheme: 1(a) I Three masses recorded and two burette readings and titre for the rough titration and initial and final readings for two (or more) accurate titrations (minimum 2 × 2 box) 1 II All three headings and units correct for accurate titrations Headings: initial / final (burette) and reading / volume / vol or reading / volume / vol at start / finish (but not V) and volume / FA 2 and added / used or titre and Units: (cm3) or / cm3 or in cm3 [or cm3 by every entry] 1 III 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 (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); • there is only one accurate titration 1 IV The final accurate titre recorded is within 0.10 cm3 of any other accurate titre. • Do not award the mark if any ‘accurate’ burette readings (apart from initial 0) are given to zero dp. 1 For assessment of accuracy (Q marks) the examiner should round any burette reading to the nearest 0.05 cm3. Subtractions should be checked. The ‘best’ titres should be selected using the hierarchy: two (or more) identical; then 2 (or more) within 0.05 cm3; then two (or more) within 0.1 cm3 etc. These best titres should be used to calculate the mean corrected titre to the nearest 0.01 cm3. Calculate mean titre/mass used to 2 dp, calculate the supervisor titre/mass, calculate the difference, δ, and award accuracy (Q) marks as shown below. 1(a) V, VI and VII Award V, VI and VII for a difference from supervisor within 0.10 Award V and VI for a difference from supervisor 0.10 < δ ⩽ 0.20 Award V for a difference from supervisor 0.20 < δ ⩽ 0.30 3 Question Answer Marks 1(b) Candidate must average two (or more) for which the total spread is not greater than 0.2 cm3. Working must be shown or ticks must be put next to the two (or more) accurate readings selected. The mean should normally be quoted to 2 dp rounded to the nearest 0.01. Example: 26.667 must be rounded to 26.67. Two special cases where the mean may not be to 2 dp: allow mean to 3 dp only for 0.025 or 0.075 e.g. 26.325; 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 correct but 26.0 and 26.1 = 26.1 is incorrect. Do not award this mark if: any selected titre is not within 0.20 cm3 of any other selected titre; the rough titre was used to calculate the mean; the candidate carried out only 1 accurate titration; burette readings were incorrectly subtracted to obtain any of the accurate titre values. All burette readings, excluding initial 0, (resulting in titre values used in calculation of mean) are integers. 1 Note: the candidate’s mean will sometimes be marked as correct even if it is different from the mean calculated by the examiner for the purpose of assessing accuracy. 1(c)(i) Answers to (c)(ii)–(iv) correct to 3 or 4 sig figs. (minimum of 3 answers attempted) 1 1(c)(ii) Correctly calculates moles acid = (b) × 0.110 / 1000 1 1(c)(iii) Na2CO3(aq) + 2HCl(aq) → 2NaCl(aq) + CO2(g) + H2O(l) and correctly uses moles sodium carbonate = (c)(ii) ÷ 2 1 1(c)(iv) Correctly uses (c)(iii) × 10 and Mr = mass from (a) ÷ (answer to (c)(iii) × 10) 1 1(c)(v) 106 18 r M − and answer as integer from working 1 1(d) Percentage error increases as volume is smaller. 1 Half the volume of sulfuric acid is needed 1

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2 In Question 1 you used a titration method to investigate a hydrated compound. In Question 2 you will use a method involving measuring masses. You will find the identity of a Group 2 element, Y, whose hydrated sulfate has the formula YSO4.7H2O. When heated, the hydrated sulfate loses its water of crystallisation to form anhydrous sulfate. The anhydrous sulfate does not decompose at the temperature of the Bunsen flame. YSO4.7H2O(s) YSO4(s) + 7H2O(g) FA 4 is the hydrated sulfate of Y, YSO4.7H2O. (a) Method ● Weigh the crucible with its lid and record the mass. ● Tip between 1.80 g and 2.00 g of FA 4 into the crucible. Keep the remaining FA 4 for Question 3. ● Weigh and record the mass of crucible, lid and FA 4. ● Place the crucible on the pipe-clay triangle on the tripod. Put the lid on the crucible and heat gently for about 1 minute. ● Use tongs to remove the lid and heat the crucible strongly for about 4 minutes. Replace the lid and then leave to cool. ● While the crucible is cooling, begin work on Question 3. ● When cool, reweigh the crucible with its lid and contents and record the mass. ● Calculate and record the mass of FA 4 before heating, the mass of residue after heating and the mass of water lost. [4] (b) Calculations (i) Calculate the number of moles of water lost on heating FA 4. moles of H2O lost = .............................. mol [1] (ii) Deduce the number of moles of anhydrous YSO4 that are formed when this water is lost. moles of YSO4 = .............................. mol [1] (iii) Use your answer to (ii) and the mass of residue left after heating FA 4 to determine the relative atomic mass, Ar, of Y. Ar of Y = .............................. [2] (iv) Identify Y. Y is .............................. [1] (c) A student did not heat the sample of FA 4 for long enough to remove all the water. What would be the effect of this on the calculated value of the relative atomic mass of Y? Explain your answer. .................................................................................................................................................... .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [2] [Total: 11] 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) Data recorded (in table) showing all headings and units • Mass of crucible + lid • Mass of crucible + lid + FA 4 • Mass of crucible + lid + residue / anhydrous FA 4 • Mass of FA 4 • Mass residue / anhydrous FA 4 • Mass water (lost) and the 3 balance readings clearly recorded to consistent number of dp. Do not allow weight. 1 Mass between 1.80 g and 2.00 g used and masses water, hydrated and anhydrous solid correctly calculated 1 Question Answer Marks For assessment of accuracy (Q) marks the examiner should check and correct the masses of water and anhydrous solid and calculate ratio of mass water/mass anhydrous solid correct to 2 dp. (theoretical value 1.05) 2(a) Award 2 marks if ratio 0.95–1.10 Award 1 mark if ratio 0.80–1.15 2 2(b)(i) Correctly calculates: mass water for (a) ÷ 18, to 2 – 4 sf 1 2(b)(ii) Correctly uses (b)(i) ÷ 7 and answer to 2 – 4 sf 1 2(b)(iii) Expression: Mr = mass anhydrous solid ÷ (b)(ii) 1 Expression: Ar = Mr – 96.1 1 2(b)(iv) Identification of Y as Group 2 element with Ar nearest to (b)(iii) 1 2(c) Moles / mass water too small / not all water lost so Ar too large «.. 1 «. as moles YSO4 too small 1

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3 (a) In Question 2 you used a gravimetric method to identify the cation, Y, present in FA 4. You will now use a qualitative analysis method to confirm whether your identification of Y was correct. Transfer a spatula measure of FA 4 into a boiling tube. Add a 5 cm depth of distilled water and shake the tube to dissolve the solid. (i) Use 1 cm depths of this solution in test-tubes to carry out tests to identify the cation, Y, present in FA 4. Record your tests and observations in a suitable form in the space below. [3] (ii) Do your qualitative analysis tests in (i) confirm your identity of Y in Question 2? Explain your answer. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [1] (b) FA 5 contains two of the ions listed in the Qualitative Analysis Notes. (i) Place a small spatula measure of FA 5 into a hard-glass test-tube and heat, gently at first and then strongly. Record your observations. observations ........................................................................................................................ ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (ii) Transfer the remaining FA 5 into the 100 cm3 beaker. Add approximately 20 cm3 of distilled water and stir to form a solution. For each of the tests below use a separate 1 cm depth of this solution in a test-tube. Record your observations. test observations Add aqueous sodium hydroxide. Add a few drops of acidified potassium manganate(VII), then add a few drops of ammonium thiocyanate. Tip the contents of the tube down the sink and rinse the tube and sink with tap water. Add a 1 cm depth of hydrogen peroxide, then add aqueous sodium hydroxide. Add a 1 cm depth of dilute nitric acid and then a few drops of aqueous silver nitrate. Add a few drops of aqueous barium nitrate or aqueous barium chloride, then add dilute hydrochloric acid. [6] (iii) Identify the ions present in FA 5. ions present ...................................................... and ..................................................... [1] (iv) What type of reaction is occurring when FA 5 reacts with acidified potassium manganate(VII)? ....................................................................................................................................... [1] [Total:14]

Mark scheme: 3(a)(i) Chooses NaOH and NH3 1 Uses both reagents in excess 1 White ppt and insoluble in excess for both 1 3(a)(ii) If Mg(2+) in Q2 then (observations) confirm it (owtte) or If other cation / metal identified in Q2 then not confirmed as (FA 4 contains) Mg2+ / magnesium (owtte) 1 3(b)(i) Observations (4 possible): • Water vapour / condensation • Gas turns litmus red • White smoke (not white fumes / white gas) • Brown / yellow / grey residue Any 3 obs = 2 marks, any 2 obs = 1 mark 2 Question Answer Marks 3(b)(ii) test observations + NaOH Green ppt and insol in excess / turns brown [1] + KMnO4 Purple to colourless / yellow [1] + NH4SCN Red / blood-red / blood-orange (solution) [1] +H2O2 Fizz and gas relights glowing splint [1] then + NaOH Brown / rust / red-brown ppt and insol in excess [1] + AgNO3 No visible reaction / no change / no ppt (ignore faint white ppt) and + Ba(NO3)2 White ppt and then + HCl (ppt) insoluble [1] 6 3(b)(iii) Fe2+ and SO4 2‒ 1 3(b)(iv) redox / Fe2+ is oxidised / MnO4 – is reduced 1

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