Cambridge A Level Chemistry 9701 — 2020 May/June Paper 3 · Variant 4

1 FB 1 is a solution of hydrated sodium carbonate, Na2CO3•xH2O, where x is an integer. You will determine the value of x in this compound by a titration method. You will add FB 1 to a known volume and concentration of hydrochloric acid, FB 2. The hydrochloric acid is in excess. You will then titrate the remaining acid with aqueous sodium hydroxide, FB 3. FB 1 is a solution containing 37.5 g dm–3 hydrated sodium carbonate, Na2CO3•xH2O. FB 2 is 0.200 mol dm–3 hydrochloric acid, HCl. FB 3 is 0.100 mol dm–3 sodium hydroxide, NaOH. bromophenol blue indicator (a) Method ●● Fill the burette with FB 3. ●● For each titration: Use the 25 cm3 pipette to place 25.0 cm3 of FB 2 into a conical flask. Use the 10 cm3 pipette to place 10.0 cm3 of FB 1 into the same conical flask. ●● Add a few drops of bromophenol blue indicator. ●● Titrate the contents of the conical flask with FB 3 from the burette. ●● 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 FB 3 added in each accurate titration. I II III IV V VI VII [7] (b) From your titration results, obtain a suitable value to be used in your calculations. Show clearly how you obtained this value. volume of FB 3 used in titration = .............................. cm3 [1] (c) Calculations (i) Give your answers to (c)(ii), (c)(iii) and (c)(iv) to an appropriate number of significant figures. [1] (ii) Calculate the number of moles of sodium hydroxide present in the volume of FB 3 calculated in (b). moles of NaOH = .............................. mol This number of moles of sodium hydroxide neutralises the remaining hydrochloric acid. Deduce the number of moles of remaining hydrochloric acid. moles of remaining HCl = .............................. mol [1] (iii) Calculate the initial number of moles of hydrochloric acid in each 25.0 cm3 sample of FB 2 pipetted into the conical flask. initial moles of HCl in each sample of FB 2 = .............................. mol You have calculated ●● the number of moles of remaining HCl ●● the initial number of moles of HCl in each sample of FB 2. Calculate the number of moles of hydrochloric acid neutralised by the Na2CO3•xH2O in FB 1 in each titration. number of moles of HCl neutralised by the Na2CO3•xH2O = .............................. mol [1] (iv) Complete the equation. Include state symbols. ...... Na2CO3(aq) + ......HCl ..... ....................... + ....................... + ....................... Deduce the number of moles of Na2CO3•xH2O present in each 10.0 cm3 sample of FB 1. number of moles of Na2CO3•xH2O present = .............................. mol [1] (v) FB 1 contains 37.5 g dm–3 Na2CO3•xH2O. Use your answer to (c)(iv) to calculate the relative formula mass, Mr, of Na2CO3•xH2O. Show your working. relative formula mass of Na2CO3•xH2O = .............................. [2] (vi) Determine the value of x in the formula Na2CO3•xH2O. x = .............................. [1] (d) A student suggested a different method. ●● To 250 cm3 of FB 2, add 100 cm3 of FB 1. ●● Pipette 25.0 cm3 of this mixture of solutions into a conical flask. ●● Titrate this mixture of solutions with FB 3. ●● Repeat titrations until concordant results are achieved. Comment on one disadvantage or one advantage of using this method rather than the method you used in (a). .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [1] [Total: 16]

Mark scheme: 1(a) I Initial and final burette readings and volume added recorded for rough titre and accurate titre details tabulated. 1 II Initial and final burette readings recorded and vol of FB 3 added recorded for each accurate titration. Headings and units correct for accurate titrations. Headings: initial / final (burette) reading / volume or reading / volume at start / finish and volume / FB 3 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. 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 > 50(.00) 1 IV The final accurate titre recorded is within 0.10 cm3 of any other accurate titre. 1 1(a) V, VI and VII Award V, VI and VII for δ ⩽ 0.30 cm3 Award V and VI for 0.30 cm3 < δ ⩽ 0.50 cm3 Award V for 0.30 cm3 < δ ⩽ 0.80 cm3 3 1(b) Calculation of mean Candidate must average two (or more) accurate 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. 1 1(c)(i) all answers to (ii), (iii), (iv) and (v) to 3–4 sf (minimum 4 answers) 1 1(c)(ii) Correctly calculates moles NaOH = × 0.100 1000 (b) to 3–4 sf and same answer for moles of HCl 1 Question Answer Marks 1(c)(iii) Correctly calculates moles HCl = × 25.0 0.200 1000 = 5.00 × 10−3 and moles neutralised = moles HCl above – moles HCl in (i) 1 1(c)(iv) Na2CO3(aq) + 2HCl (aq) → 2NaCl(aq) + CO2(g) + H2O(l) and correctly uses moles Na2CO3 = moles in HC in 2 (ii) l 1 1(c)(v) ans (iii) × 100 37.5 / ((iii) × 100) can be done in either order moles = × 37.5 100 (iii) = 2 marks any error loses 1 mark 1 1(c)(vi) Correctly calculates x ((Mr –106) / 18) and gives to nearest integer allow ecf even if negative integer. 1 1(d) Disadvantage: if make mistake all the solution is wrong Advantage: quicker/ larger volume has less % error 1

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

2 You will now investigate a different hydrated salt with the formula MSO4•7H2O, where M is a Group 2 metal. By heating a sample of MSO4•7H2O to produce anhydrous MSO4 you will determine its relative formula mass and hence identify M. FB 4 is the hydrated salt MSO4•7H2O. (a) Method ●● Weigh the crucible with its lid. Record the mass. ●● Place between 1.80 g and 2.20 g of FB 4 in the crucible. ●● Reweigh the crucible, its lid and contents and record the mass. ●● Without the lid, place the crucible on the pipe-clay triangle and heat gently for approximately 1 minute and then strongly for approximately 4 minutes. ●● Place the lid on the crucible and leave it to cool. ●● You may wish to start Question 3 while you are waiting for the crucible to cool. ●● Reweigh the crucible, its lid and contents and record the mass. ●● Calculate, and record, the mass of FB 4, the mass of residue after heating and the mass of water lost. [4] (b) Calculations (i) Calculate the number of moles of water lost when your sample of MSO4•7H2O was heated. moles of water = .............................. mol [1] (ii) Write the equation for the reaction that occurs when MSO4•7H2O is heated. Include state symbols. ............................................................................................................................................. Deduce the number of moles of anhydrous salt, MSO4, left after the heating. moles of MSO4 = .............................. mol [1] (iv) Determine the relative atomic mass, Ar, of M and hence identify M. Show your working. Ar = .............................. M is .............................. [2] (c) (i) In the method used above, the lid was placed on the crucible when the crucible was left to cool. Explain why the lid was placed on the crucible. ............................................................................................................................................. ....................................................................................................................................... [1] (ii) Suggest and explain the effect on the calculated value of the relative atomic mass of M if the lid had not been placed on the crucible during cooling. ............................................................................................................................................. ....................................................................................................................................... [1] [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) I Table with headings and units for 3 weighings and 3 calculated masses 1 II Mass FB 4 used between 1.80 and 2.20 g masses of FB 4, residue and water lost correctly calculated. 1 III Ratio within 1.95–2.15 of supervisor ratio 1 III and IV Ratio within 2.00–2.10 of supervisor ratio 1 2(b)(i) Correctly calculates moles water = mass water 18 to 2–4sf Penalise sf only once in this question. 1 2(b)(ii) MSO4.7H2O(s) → MSO4(s) + 7H2O(g) and correctly uses moles anhydrous = (ii) / 7 1 2(b)(iii) Correctly uses mass 4 moles anhydrous FB = Mr 1 2(b)(iv) Shows Ar M + 222.1 = Mr FB 4 and calculates Ar of M 1 identifies M as Group 2 metal with nearest Ar 1 2(c)(i) To avoid water being picked up (from air) 1 2(c)(ii) Ar higher with explanation Mass water lost less → moles water lost less → moles anhydrous more → Mr higher → Ar higher very little effect because very little water vapour absorbed if lid prevented loss in (i) then allow Ar lower as ecf 1

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

3 In this question you may need Tollens’ reagent. To prepare this, place a 2–3 cm depth of aqueous silver nitrate in a test‑tube, add aqueous sodium hydroxide drop by drop until a small amount of brown precipitate is formed and then add aqueous ammonia drop by drop with shaking until the precipitate just dissolves. This is Tollens’ reagent. If Tollens’ reagent is used, ensure that all test‑tubes are thoroughly rinsed immediately after use. Half fill the 250 cm3 beaker with water and heat to boiling. Then turn off the Bunsen burner. This will be used as a hot water bath. (a) (i) You are to investigate some reactions of solid FB 5. To a 2 cm depth of aqueous ammonium vanadate(V) in a test‑tube add a small spatula measure of FB 5. Leave for approximately 4 minutes with occasional shaking. Record all the changes that you observe. Keep the test‑tube and its contents for use in the next test. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (ii) Transfer a 1 cm depth of the solution from (a)(i) into a test‑tube and add acidified potassium manganate(VII) a few drops at a time until no further reaction occurs. At this stage the solution is pink because unreacted KMnO4 is present. Record all the changes that you observe. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [1] (iii) State the type of reaction occurring in the test in (a)(ii). ....................................................................................................................................... [1] (iv) To a 1 cm depth of dilute sulfuric acid in a test‑tube add a small spatula measure of FB 5. Record your observations. Place the test-tube in the hot water bath if necessary to start the reaction. ............................................................................................................................................. ....................................................................................................................................... [2] (b) FB 6 is an aqueous solution that has been made by reacting solid FB 5 with dilute sulfuric acid. (i) Carry out the following tests and record your observations. test observations Test 1 To a 1 cm depth of FB 6 in a test‑tube add aqueous sodium hydroxide. Test 2 To a 1 cm depth of FB 6 in a test‑tube add aqueous ammonia. [2] (ii) Identify FB 5. FB 5 is ..................................... . [1] (iii) Give the equation for the reaction of FB 5 with sulfuric acid to make FB 6. Include state symbols. ....................................................................................................................................... [1] (c) FB 7 is either ethanal, CH3CHO, or propanone, CH3COCH3. (i) Describe a test that would enable you to identify which of these compounds is present in FB 7. You should state the observation expected for ethanal and propanone. test ...................................................................................................................................... ............................................................................................................................................. expected observations ethanal ................................................................................................................................ propanone ........................................................................................................................... [2] (ii) Carry out this test on FB 7. Record the result of the test and hence identify FB 7. result ................................................................................................................................... FB 7 is .................................... . [1] [Total: 13]

Mark scheme: 3(a)(i) (Yellow solution goes) to blue / green 1 then to lilac / violet / mauve 1 3(a)(ii) any two from; • blue (ignore green if before blue) • green / yellow • KMnO4 loses purple colour 1 3(a)(iii) Redox 1 3(a)(iv) Fizz / bubbles / effervescence 1 Gas / H2 (gas) burns with a pop / pops with a lighted splint 1 3(b)(i) test observation FB 6 + NaOH white ppt* soluble in excess* FB 6 + NH3 white ppt* soluble in excess * Award one mark for every two correct observations (*) 2 3(b)(ii) FB 5 is Zn / zinc (metal) 1 3(b)(iii) Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g) ecf for incorrect FB 5 provided gas is produced. 1 Question Answer Marks 3(c)(i) Choice of reagent: Tollens’ and warming / Fehling’s and warming / Benedict’s and warming / Sandell’s and warming / acidified potassium manganate(VII) 1 Positive test for ethanal – silver mirror / black (or dark grey) solid for Tollens’ Red / brown (solid / ppt) for Fehling’s / Benedict’s / Sandell’s Purple to colourless for acidified manganate(VII) negative result for propanone – no reaction 1 3(c)(ii) Negative test result and FB 7 is propanone 1

More questions on Redox processes: electron transfer and changes in oxidation number (oxidation state)