Cambridge A Level Chemistry 9701 — 2023 Oct/Nov Paper 3 · Variant 5

1 You will determine the value of x in hydrated sodium carbonate, Na2CO3•xH2O. x is not an integer. You will carry out two methods to determine the value of x. Each method involves sodium carbonate reacting with excess hydrochloric acid to release carbon dioxide. Na2CO3•xH2O(s) + 2HCl (aq) → 2NaCl (aq) + CO2(g) + xH2O(l) (a) Experiment 1 You will measure the volume of carbon dioxide released when hydrated sodium carbonate reacts with excess hydrochloric acid. FA 1 is 0.500 mol dm–3 hydrochloric acid, HCl. FA 2 is hydrated sodium carbonate, Na2CO3•xH2O. Method ● Weigh the container with FA 2. Record the mass. ● Fill the tub with water to a depth of approximately 5 cm. ● Fill the 250 cm3 measuring cylinder completely with water. Holding a piece of paper towel firmly over the top, invert the measuring cylinder and place it in the water in the tub. ● Remove the paper towel and clamp the inverted measuring cylinder so the open end is in the water just above the base of the tub. ● Using the 50 cm3 measuring cylinder, transfer 50.0 cm3 of FA 1 into the flask labelled Z. Check that the bung fits tightly into the neck of flask Z, clamp flask Z and place the end of the delivery tube into the inverted 250 cm3 measuring cylinder. ● Remove the bung from the neck of the flask. Tip all the FA 2 from the container into the acid in the flask and replace the bung immediately. Remove the flask from the clamp and swirl it to mix the contents. You may need to shake the flask quite vigorously until the gas formed starts to collect in the measuring cylinder. ● Return the flask to the clamp. Leave for several minutes, shaking the flask occasionally. ● Weigh the container with any residual FA 2. Record the mass. ● Calculate the mass of FA 2 added to the flask. Record the mass. ● When no more gas is collected, measure the final volume of gas in the measuring cylinder. Record the volume. Results I II III [3] (b) Calculations (i) Calculate the amount, in mol, of carbon dioxide collected in the measuring cylinder at room conditions. amount of CO2 = .............................. mol Hence deduce the amount, in mol, of sodium carbonate present in the FA 2 you added in your experiment. amount of Na2CO3 = .............................. mol [1] (ii) Use your answer to (b)(i) and the mass of hydrated sodium carbonate, Na2CO3•xH2O, you used in Experiment 1 to calculate the relative formula mass, Mr, of the Na2CO3•xH2O. Mr of Na2CO3•xH2O = .............................. [1] (iii) Use your answer to (b)(ii) to calculate the value of x in the Na2CO3•xH2O. Show your working. x = .............................. [2] (c) A student suggests that it would be better to use hot water in the tub. (i) State whether using hot water would be an improvement. Explain your answer. ........................................................................................................................................... ..................................................................................................................................... [1] (ii) State the effect, if any, that using hot water would have on the value of x calculated. ........................................................................................................................................... ..................................................................................................................................... [1] (d) Experiment 2 You will carry out a titration to measure the volume of hydrochloric acid that neutralises an aqueous solution of hydrated sodium carbonate, Na2CO3•xH2O. Na2CO3•xH2O(s) + 2HCl (aq) → 2NaCl (aq) + CO2(g) + xH2O(l) FA 3 is 0.100 mol dm–3 hydrochloric acid, HCl. FA 4 is an aqueous solution containing 14.30 g dm–3 of hydrated sodium carbonate, Na2CO3•xH2O. FA 5 is bromophenol blue indicator. Method ● Fill the burette with FA 3. ● Pipette 25.0 cm3 of FA 4 into a conical flask. ● Add a few drops of FA 5. ● Carry out 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 your 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 3 added in each accurate titration. I II III IV V VI VII [7] (e) 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 4 required .............................. cm3 of FA 3. [1] (f) Calculations (i) Give your answers to (f)(ii), (f)(iii) and (f)(iv) to an appropriate number of significant figures. [1] (ii) Calculate the amount, in mol, of hydrochloric acid present in the volume of FA 3 you calculated in (e). amount of HCl = .............................. mol [1] (iii) Use the equation for the neutralisation to deduce the amount, in mol, of sodium carbonate present in 25.0 cm3 of Na2CO3•xH2O. amount of Na2CO3 = .............................. mol Hence calculate the amount, in mol, of sodium carbonate in 1.00 dm3 of Na2CO3•xH2O. amount of Na2CO3 in 1.00 dm3 = .............................. mol [1] (iv) Calculate the value of x in the sample of Na2CO3•xH2O. Show your working. x = .............................. [1] (g) The aqueous solution of Na2CO3•xH2O, FA 4, was prepared by weighing and dissolving the solid to make 1.00 dm3 of solution. Mass of container + Na2CO3•xH2O = 32.509 g Mass of empty container = 18.209 g Mass of Na2CO3•xH2O = 14.300 g (i) State the maximum uncertainty in a single balance reading for the balance used. maximum uncertainty = ± .............................. g Calculate the maximum percentage uncertainty in this mass of Na2CO3•xH2O. Show your working. maximum percentage uncertainty = ± .............................. % [1] (ii) Using the method in Experiment 2 a student calculated the relative formula mass, Mr, of Na2CO3•xH2O to be 242.2. Assume that the uncertainty in the mass of FA 4 is the only source of error in the experiment. Calculate the maximum value for the relative formula mass of FA 4. maximum value for the relative formula mass of FA 4 = .............................. [1] [Total: 23] 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: Question Answer Marks 1(a) I Appropriate headings and units for recorded data in the space provided. 3 (Mass of) container + FA 2 (Mass of) container (+ residual FA 2) (Mass of) FA 2 (used) (Volume of) carbon dioxide Mass units: / g or in g or (g) or g by every reading Volume units: / cm3 or in cm3 or (cm3) or cm3 by every reading II Balance readings recorded to same number of dp (2 or 3) and volumes given as integers and correctly calculates: mass of solid / FA2 used and volume of gas collected III Volume of gas collected in range 100–200 cm3 1(b)(i) Correctly calculates 1 amount CO2 = volume of gas / 24000 and amount of Na2CO3 is the same Both answers correctly rounded to 2–4 sf 1(b)(ii) Correctly uses 1 Mr = mass of FA 2 / amount Na2CO3 from (i) Answer to 2–4 sf 1(b)(iii) Correctly uses 2 M1: Mr of xH2O = Mr (from (ii)) – 106 M2: x = Mr of xH2O / 18 and Final answer to 2–4 sf 1(c)(i) It is an improvement and carbon dioxide is less soluble (if water is hot) 1 1(c)(ii) Amount of carbon dioxide / sodium carbonate is more, (Mr is less) so x is smaller 1 1(d) I The following data must be shown • two burette readings and titre for rough titration • initial and final burette readings for two (or more) accurate titrations II Titre values shown for accurate titrations and appropriate headings and units in the accurate titration table • initial / start and (burette) reading / volume • final / end and (burette) reading / volume • titre or volume / FA 3 and used / added unit: /cm3 or (cm3) or in cm3(for each heading) or cm3 unit given for each volume recorded III All accurate burette readings are to nearest 0.05 cm3. IV The final accurate titre recorded is within 0.10 cm3 of any other accurate titre. Round burette readings to the nearest 0.05 cm3. Check and correct titre subtractions where necessary. Examiner selects the best mean titre. Apply hierarchy: 2 identical, titres within 0.05 cm3, titres within 0.10 cm3, etc. Examiner subtracts (corrected) candidate’s titre from Supervisor’s titre, . Award the accuracy (Q) marks as follows: 7 V, VI, VII Award V if  ⩽ 0.50 cm3 Award VI if  ⩽ 0.30 cm3 Award VII if  ⩽ 0.20 cm3 1(e) Candidate calculates mean correctly to 2 dp 1 • 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 the nearest 0.01 cm3. 1(f)(i) Answers to (ii) and (iii) given to 3–4 sf 1 1(f)(ii) Correctly calculates 1 Amount of HCl = 0.1  volume (e) / 1000 1(f)(iii) Correctly uses 1 Amount Na2CO3•xH2O = (ii) / 2 and multiply by 40 1(f)(iv) Correctly uses 1 Mr = 14.30 / (iii) Mr of xH2O = Mr – 106 x = Mr of xH2O / 18 1(g)(i) Correct expression 1 Uncertainty in a single reading =  0.001 or 0.0005 and 2  uncertainty given  100 14.3 1(g)(ii) Correctly uses 1 (100 + final answer to (g)(i)) / 100  242.2 or (14.3 + 2  (single) uncertainty) / answer f(iii)

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2 (a) FA 6, FA 7 and FA 8 are salts each of which contains nitrogen. Each of the nitrogen-containing ions is different and all are shown in the Qualitative analysis notes. (i) List the nitrogen-containing ions for which you will test. ......................... and ......................... and ......................... . Select reagents to positively identify the nitrogen-containing ions in each salt. Record your tests and the results with each salt in a suitable table in the space below. [8] (ii) Use your observations in (a)(i) to determine the formulae of the nitrogen-containing ions present in FA 6, FA 7 and FA 8. FA 6 ......................... FA 7 ......................... FA 8 ......................... [1] (b) FA 9 and FA 10 contain the same element. You will identify this element by carrying out tests. (i) Test 1 Heat a small spatula measure of FA 9 in a hard-glass test-tube. Heat until the reaction stops. After heating, leave the tube to cool and keep it for Test 2. You may wish to start (b)(ii) while you wait. Record your observations. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... Name one product of the reaction. product .............................. Test 2 To the cooled solid product of Test 1, add a 2–3 cm depth of distilled water. Shake the test-tube and then leave the contents to settle. Record your observations. ........................................................................................................................................... ........................................................................................................................................... [3] (ii) To a very small spatula measure of FA 9 in a test-tube, add about a 2 cm depth of dilute sulfuric acid and about a 2 cm depth of distilled water. Shake to dissolve the FA 9 and produce FA 9(aq). You will use FA 9(aq) in Test 3 and Test 4. Test 3 To a 1 cm depth of aqueous iron(II) sulfate in a test-tube, add a few drops of FA 9(aq). Record your observations. ........................................................................................................................................... ........................................................................................................................................... Test 4 To a 1 cm depth of aqueous potassium iodide in a test-tube, add a few drops of FA 9(aq). Record your observations. ........................................................................................................................................... ........................................................................................................................................... [2] (iii) To a small spatula measure of FA 10 in a test-tube, add distilled water to dissolve the FA 10. This solution is FA 10(aq). To a 1 cm depth of FA 10(aq), add aqueous sodium hydroxide. Record your observations. ........................................................................................................................................... ..................................................................................................................................... [2] (iv) Identify the element that is present in FA 9 and FA 10. element .............................. [1] [Total: 17]

Mark scheme: 2(a)(i) M1: Correct three ions listed in space provided 8 M2: Table to show FA6 / FA7 / FA8 and test reagents M3: NaOH and heat listed as a reagent M4: Only FA8 gives off a gas that turns (damp red) litmus blue M5: Add Al to NaOH and heat M6: FA6 gives off a gas that turns (damp red) litmus blue M7: To a new sample add either (acidified) KMnO4 or named acid M8: Only FA7 either turns KMnO4 colourless or gives off a brown gas with a named acid Expected observations for reference: reagent FA6 FA7 FA8 NaOH (aq) and heat No (visible) reaction / No (visible) reaction / gas test with (damp No fizz No fizz red) litmus to blue Then gas test with gas test with (damp Ignore add Al (damp red) litmus to red) litmus to blue blue (Acidified) KMnO4 Purple remains / Purple to colourless Purple remains / (heat with solution) No decolouration No decolouration No (visible) reaction / No (visible) reaction or named acid No fizz Brown gas No fizz 2(a)(ii) FA 6 is NO3− FA 7 is NO2− FA 8 is NH4+ 1 2(b)(i) Test 1 3 6 * available. 2 * = 1 mark. Round down. Observations Black solid/residue * Pops / jumps about (OWTTE) * Glowing splint relights * Product Oxygen / manganese(IV) oxide / potassium manganate(VI) * Test 2 (Dark) green * solution * 2(b)(ii) M1: Test 3 2 Purple to colourless / pale yellow / yellow-brown (solution) M2: Test 4 Purple (to colourless) to brown (solution) 2(b)(iii) M1: Off -white ppt 2 M2: Ppt turns brown(er) (on standing) and insoluble in excess (NaOH(aq)) 2(b)(iv) Manganese / Mn 1

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