Cambridge A Level Chemistry 9701 — 2021 May/June Paper 3 · Variant 2

1 Washing soda consists of hydrated sodium carbonate, Na2CO3•10H2O. When it is stored it loses some of its water of crystallisation to leave Na2CO3•xH2O. Since water has been lost x is no longer an integer. You will carry out a titration to determine the value of x. You will titrate a solution of the sodium carbonate with hydrochloric acid. The equation for the reaction is shown. Na2CO3•xH2O(aq) + 2HCl (aq) → 2NaCl (aq) + CO2(aq) + (x+1)H2O(l) FB 1 is an aqueous solution containing 11.30 g dm–3 of Na2CO3•xH2O. FB 2 is 0.100 mol dm–3 hydrochloric acid, HCl. bromophenol blue indicator (a) Method ● Fill the burette with FB 2. ● Pipette 25.0 cm3 of FB 1 into a conical flask. ● Add a few drops of bromophenol blue indicator. ● 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. ● Make sure your recorded results show the precision of your practical work. ● Record in a suitable form, in the space below, all of your burette readings and the volume of FB 2 added in each accurate titration. I II III IV V VI VII [7] (b) From your accurate titration results, obtain a value for the volume of FB 2 to be used in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FB 1 required .............................. cm3 of FB 2. [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 hydrochloric acid present in the volume of FB 2 you calculated in (b). moles of HCl = .............................. mol [1] (iii) Use the equation on page 2, and your answer to (c)(ii), to calculate the concentration, in mol dm–3, of Na2CO3•xH2O present in FB 1. concentration of Na2CO3•xH2O = .............................. mol dm–3 [1] (iv) Calculate the value of x in this sample of Na2CO3•xH2O. Show your working. x = .............................. [3] [Total: 14]

Mark scheme: 1(a) I Initial and final burette readings and volume added recorded for rough titre and accurate titre details tabulated. (minimum 2 × 2 boxes with relevant information). 1 II Initial and final burette readings recorded and vol of FB 2 added recorded for each accurate titration. Headings and units correct for accurate titrations. Headings: initial / final and (burette) reading / volume or reading / volume at start / finish Do not allow difference / change and volume / FB 2 added / used or titre and units (cm3) or / cm3 or in cm3 or cm3 by every entry Accept vol but not V 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 0.1 cm3 of any other accurate titre. Do not award the mark if any accurate burette readings (apart from 0) are given to zero dp. 1 For assessment of accuracy (Q) marks, the Examiner should round any burette readings to the nearest 0.05 cm3. Check and correct subtractions. 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 difference (δ) between the candidate’s titre and the supervisor’s titre. Award the accuracy (Q) marks as shown below. Question Answer Marks 1(a) V, VI and VII Award V, VI and VII for δ ⩽ 0.20 cm3 Award V and VI for 0.20cm3 < δ ⩽ 0.30 cm3 Award V for 0.30 cm3 < δ ⩽ 0.50 cm3 3 1(b) Candidate must average two (or more) titres that are all within 0.20 cm3. Working must be shown or ticks must be put next to the two (or more) accurate titres selected. 1 1(c)(i) Answers to (ii) and (iii) given to 3–4 sf. 1 1(c)(ii) Correctly calculates moles of HCl = × 0.1 vol in 1000 (b) 1 1(c)(iii) Correctly uses (c)(ii) ÷ 2 and x 40 1 1(c)(iv) Correctly uses M1: Mr = 11.30 moles from (c)(iii) M2: xH2O = Mr hydrated −106 or Mr = 106 + 18x M3: xH2O to calculate x = M2 / 18 and answer shown to 2–4 sf ALLOW alternative methods (e.g. ratio of anhydrous : water) 3

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2 When anhydrous sodium carbonate dissolves in water the following reaction occurs. Na2CO3(s) + aq → 2Na+(aq) + CO32–(aq) The enthalpy change for this reaction is the enthalpy change of solution of anhydrous sodium carbonate. Its value is –28.1 kJ mol–1. You will use this information to find the percentage purity of an impure sample of anhydrous sodium carbonate. FB 3 is a sample of impure anhydrous sodium carbonate. (a) Method Experiment 1 ● Weigh the cup and record the mass. ● Place between 1.9 g and 2.1 g of FB 3 in the cup. Record the mass of the cup + FB 3. ● Calculate and record the mass of FB 3 used. ● Support the cup in the beaker. ● Place 25.0 cm3 of distilled water into the measuring cylinder. Measure and record the temperature of the water. ● Place the water in the cup and stir until all the solid dissolves. ● Place the thermometer in the solution and record the highest temperature reached. Tilt the cup if necessary so that the bulb of the thermometer is fully covered. ● Rinse the cup and shake dry ready to carry out Experiment 2. Experiment 2 ● Repeat the experiment using between 3.9 g and 4.1 g of FB 3 and 25.0 cm3 of water. ● Record your results in the same way as in Experiment 1. Results I II III IV V [5] (b) Calculations (i) Use your results from Experiment 1 to calculate the heat energy, in J, released when the FB 3 dissolves. (Assume 4.2 J of heat energy changes the temperature of 1.0 cm3 of solution by 1.0 °C.) heat energy = .............................. J [1] (ii) Use the information on page 4 to calculate the number of moles of anhydrous sodium carbonate needed to release the amount of energy in your answer to (b)(i). moles of Na2CO3 = .............................. mol [1] (iii) Calculate the mass of anhydrous sodium carbonate present in the number of moles in your answer to (b)(ii). mass of Na2CO3 = .............................. g [1] (iv) Use your answer to (b)(iii) to calculate the percentage purity of anhydrous sodium carbonate in FB 3. percentage purity = .............................. [1] (c) In (b)(iv) you calculated the percentage purity of anhydrous sodium carbonate in FB 3. What did you assume about the impurity present? .................................................................................................................................................... .............................................................................................................................................. [1] (d) You carried out two experiments to measure the temperature change when anhydrous sodium carbonate was dissolved in water. Complete the sentence below to explain which of these experiments was more accurate. Your answer should not make reference to the fact that different masses of sodium carbonate were used. Experiment .................... was more accurate because .............................................................. .................................................................................................................................................... ............................................................................................................................................ . [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/lists given for at least one experiment Unambiguous headings and correctly displayed units • (Mass of) cup • (Mass of) cup + FB 3 • (Mass of) FB 3 • Initial thermometer reading / temperature • Maximum / highest / final thermometer reading / temperature 1 II Precision of all four readings • thermometer readings to .5 °C • balance readings to same number of dp 1 III Masses of FB 3 1.90 to 2.10 and 3.90 to 4.10 g and subtractions correct 1 IV and V Examiner to round thermometer readings to nearest .5 ºC, calculate ΔT values then calculate ratio to 1 dp: ΔT experiment 2 / ΔT experiment 1 Award IV if 1.6 to 2.4 Award V if 1.8 to 2.2 2 2(b)(i) Correctly calculates heat energy = 25 × 4.2 × ΔT to 2–4 sf 1 2(b)(ii) Correctly uses moles Na2CO3 = 28100 (b)(i) 1 2(b)(iii) Correctly uses mass Na2CO3 = (b)(ii) × 106 Allow ecf for RFM anhydrous Na2CO3 in 1(c)(iv) 1 2(b)(iv) Correctly uses = × % 100 Mass from Expt 1 (b)(iii) FB3 1 Question Answer Marks 2(c) The enthalpy of solution of the impurity was = 0 / negligible / the impurity does not give out / absorb any heat / cause a temperature change when it dissolves / impurity does not dissolve in water 1 2(d) Experiment 1 was more accurate because a smaller temperature rise meant that heat losses were less. OR Experiment 2 was more accurate because has a larger temperature rise and therefore a smaller percentage error in temperature rise. 1

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3 (a) FB 4 and FB 5 each contain one cation and one anion from those listed in the Qualitative Analysis Notes. Carry out the following tests and record your observations. observations test FB 4 FB 5 Test 1 Heat a spatula measure of solid in a hard-glass test-tube gently at first and then more strongly, then leave the tube to cool. Test 2 To a small spatula measure of solid in a boiling tube add a 2 cm depth of dilute sulfuric acid. [5] (b) FB 6 is a solution prepared by reacting FB 4 with dilute sulfuric acid. FB 7 is a solution prepared by reacting FB 5 with dilute sulfuric acid. In each test use a 1 cm depth of FB 6 or FB 7 in a test-tube. Carry out the tests and record your observations. observations test FB 6 FB 7 Test 1 Add a 1 cm depth of aqueous edta. Test 2 Add aqueous sodium hydroxide until no further reaction occurs. Test 3 Add aqueous ammonia until no further reaction occurs. Test 4 Add a 1 cm length of magnesium ribbon and leave to stand for a few minutes. Test 5 Add a 2 cm depth of aqueous potassium iodide, then add a few drops of starch indicator. Test 6 Add concentrated hydrochloric acid (CARE) drop by drop until no further change is seen. Wash the test-tube thoroughly with plenty of water once your observations are complete. [7] (c) Complete the table with the formula of each cation and each anion in FB 4 and FB 5. If you are unable to identify an ion write ‘unknown’. FB 4 FB 5 cation anion [2] (d) Write the ionic equation for the reaction between FB 6 and magnesium. Include state symbols. .............................................................................................................................................. [1] [Total: 15]

Mark scheme: 3(a) FB 4 FB 5 Test 1 heat blue / green solid * white solid * goes black * goes yellow * condensation in either * cooling ignore white / paler solid * Test 2 + H2SO4 bubbles in both * blue soln * colourless soln * attempts limewater in either * white ppt / turns milky / cloudy white * 2 * = 1 mark (round down) 5 Question Answer Marks 3(b) test FB 6 FB 7 edta turns brighter / more intense / darker blue * ALLOW light blue to dark blue No (visible) change / no reaction * ALLOW (solution) stays colourless NaOH Blue ppt and insoluble in excess * White ppt * Soluble in excess * NH3 (Pale) blue ppt * (Soluble excess to) dark blue solution * White ppt * Soluble in excess * Mg Pink / brown / dark grey / black solid * ALLOW ppt Blue fades / bubbles * KI Brown / yellow-brown / ochre * (ignore state) + starch Black / blue-black / dark blue * HCl (Solution) goes green / yellow * 2 asterisks = 1 mark (round down) 7 3(c) 2 asterisks = 1 mark (round down) FB 4 FB 5 cation Cu2+* Zn2+* anion CO32–* CO32–* 2 Question Answer Marks 3(d) Cu2+(aq) + Mg(s) → Cu(s) + Mg2+(aq) If bubbles recorded in test 4, allow Mg(s) + 2H+(aq) → Mg2+(aq) + H2(g) Allow ecf on identity of FB 6 1

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