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

1 FA 1 is an aqueous solution of a monoprotic organic acid. You will investigate the identity of FA 1 by using a titration method to find its relative molecular mass, Mr. FA 1 is an aqueous solution containing 6.20 g dm–3 of a monoprotic organic acid. FA 2 is 0.105 mol dm–3 sodium hydroxide, NaOH. thymol blue indicator (a) Method ●● Pipette 25.0 cm3 of FA 1 into a conical flask. ●● Fill the burette with FA 2. ●● Add several drops of thymol blue 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. ●● 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 added in each accurate titration. I II III IV V VI VII [7] (b) From your accurate titration results, obtain a suitable value to be used in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FA 1 required .............................. cm3 of FA 2. [1] (c) Calculations (i) Calculate the number of moles of sodium hydroxide present in the volume of FA 2 calculated in (b). moles of NaOH = .............................. mol [1] (ii) Deduce the number of moles of the organic acid present in 25.0 cm3 of FA 1. moles of organic acid = .............................. mol Hence calculate the concentration, in mol dm–3, of the organic acid in FA 1. Show your working. concentration of the organic acid = .............................. mol dm–3 [1] (iii) Calculate the relative molecular mass, Mr, of the organic acid in FA 1. Mr of the organic acid = .............................. [1] (iv) From another experiment it is found that FA 1 contains one of the following. CH3COOH HCOOH C2H4Cl COOH CH2CHCOOH NaOH(aq) reacts only with the COOH group in the acid. Deduce which of these acids is present in FA 1. Explain your answer. ............................................................................................................................................. ....................................................................................................................................... [1] (d) This method of investigation uses the relative molecular mass of the acid. The relative molecular masses of C2H5COOH and CH2CHCOOH are similar so that any inaccuracy in the practical procedure could lead to an incorrect conclusion. Suggest a chemical test that would enable you to distinguish between C2H5COOH and CH2CHCOOH. Include the test and the results expected but do not carry out this test. .................................................................................................................................................... .............................................................................................................................................. [1] (e) A student is given a solution of another organic acid containing the same concentration, in mol dm–3, as that used in (a). The student assumes this acid is monoprotic but it is diprotic. Explain the effect the student’s assumption has on the value of the relative molecular mass that the student calculates. .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [2] [Total 15]

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 FA 2 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/ FA 2 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. 1 IV The final accurate titre recorded is within 0.1 cm3 of any other accurate titre. 1 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) 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) Correctly calculates moles of FA 2 = × 0 0.105 vol i 00 n 1 (b) correct to 3–4 sf but do not penalise more than once in this question. 1 1(c)(ii) Moles in 25.0 cm3 = same as (i) and shows moles in 1 dm3 answer × 40 (3–4 sf) 1 1(c)(iii) Uses Mr = 6 f .2 moles ro 0 m (ii) 1 Question Answer Marks 1(c)(iv) Chooses acid with Mr nearest to that found and explains choice HCOOH = 46; CH2=CHCOOH = 72 C2H4ClCOOH = 111.5 1 1(d) Add bromine / acidified (or alkaline) potassium manganate(VII) and CH2CHCOOH decolourises (turns green) but C2H5COOH does not 1 1(e) (Volume of alkali doubled) → moles of acid doubled 1 → Mr halved Allow 1 for volume of alkali more → more moles acid → Mr less 1

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2 When an organic acid, RCOOH, is neutralised by an alkali an exothermic reaction takes place. You will determine the enthalpy change of neutralisation, ΔH, for the following reaction. RCOOH(aq) + NaOH(aq) RCOONa(aq) + H2O(l) In this equation R is an alkyl group. FA 3 is a solution containing 120.1 g dm–3 of RCOOH. FA 4 is aqueous sodium hydroxide, NaOH. (a) Method Experiment 1 ●● Support the cup in the 250 cm3 beaker. ●● Use the 25 cm3 measuring cylinder to transfer 25.0 cm3 of FA 3 into the cup. ●● Measure and record the temperature of this FA 3. Rinse the thermometer. ●● Place 25.0 cm3 of FA 4 into the 50 cm3 measuring cylinder. ●● Measure and record the temperature of the FA 4 in the measuring cylinder. Rinse the thermometer. ●● Tip the FA 4 from the measuring cylinder into the cup. Stir, then measure and record the highest temperature reached. ●● Calculate and record the average initial temperature of FA 3 and FA 4. ●● Calculate and record the difference between the average initial temperature and the highest temperature reached. ●● Rinse and dry the cup for use in Experiment 2. Experiment 2 ●● Repeat Experiment 1 using 50.0 cm3 of FA 3 and FA 4. You will need to use the 25 cm3 measuring cylinder twice to measure the FA 3. ●● Calculate and record the average initial temperature of FA 3 and FA 4. ●● Calculate and record the difference between the average initial temperature and the highest temperature reached. [4] (b) Calculations (i) Calculate the energy released in Experiment 1. (Assume that 4.2 J of energy changes the temperature of 1.0 cm3 of solution by 1.0 °C.) energy released = .............................. J [1] (ii) Calculate the number of moles of RCOOH used in Experiment 1. Assume that the relative molecular mass, Mr, of RCOOH is 122. Show your working. moles of RCOOH = .............................. mol [2] (iii) Calculate the enthalpy change of neutralisation, ΔH, of RCOOH. Assume that the sodium hydroxide is in excess. enthalpy change of neutralisation of RCOOH = ...... ............................. kJ mol–1 sign value [1] (c) Each measuring cylinder can be read to an accuracy of ± 0.5 cm3. Calculate the total maximum percentage error in the volumes of solution measured in each of Experiments 1 and 2. Experiment 1 total maximum percentage error = .............................. % Experiment 2 total maximum percentage error = .............................. % [2] (d) A student repeated both experiments in (a) using hydrochloric acid in place of RCOOH. Suggest how the temperature rise when using HCl would compare to the temperature rise recorded in (a). Assume all volumes and concentrations of solutions, in mol dm–3, are the same. Explain your answer by considering the chemical bonds involved. .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [2] [Total: 12] 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) Table with headings and units. Headings for initial T FA 3, initial T FA 4, average initial T, highest T, T rise. Experiments 1 and 2 clearly shown. 1 All thermometer readings to .5 °C 1 Correct calculation of average initial Ts and T rises 1 Both T rises for Experiments 1 and 2 within 1 °C of each other 1 2(b)(i) Correctly calculates 50 × 4.2 × T rise from Expt 1 and answer to 2–4 sf 1 2(b)(ii) Correctly uses moles FA 3 = 120.1 122 (= 0.9844) 1 Correctly calculates answer 40 = 0.0246 (= 0.025) mol 1 2(b)(iii) Correctly uses × 1000 (i) (ii) (to 2–4 sf) and negative sign 1 2(c) Correctly calculates Expt 1 + 0.5 0.5 25 25 × 100 = 4% 1 Expt 2 + + 0.5 0.5 0.5 50 25 25 = 5% 1 2(d) With HCl T rise more (comparison needed) 1 In organic acid some energy needed to break O–H bond / to release H+ (from the molecule) 1

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3 Half fill the beaker with water and place it on a tripod and gauze. Heat until the water begins to boil then switch off your Bunsen burner. This is the hot water bath. For a test in (a)(i) you will need Tollens’ reagent. Place a 2–3 cm depth of 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. When Tollens’ reagent is used, ensure that all test-tubes are thoroughly rinsed immediately after use. (a) FA 5, FA 6 and FA 7 are organic compounds each of which contains carbon, hydrogen and oxygen only. (i) Carry out the following tests on FA 5, FA 6 and FA 7. Use a 1 cm depth of organic compound in a test‑tube for each test. One test has been done for you. observations test FA 5 FA 6 FA 7 Test 1 no visible reaction orange precipitate orange precipitate Add 2,4‑dinitrophenylhydrazine. formed formed Test 2 Add a 1 cm length of magnesium ribbon. Test 3 Add a 1 cm depth of Tollens’ reagent, place the tube in the hot water bath and leave for a few minutes. Test 4 Add a few drops of acidified potassium manganate(VII), place the tube in the hot water bath and leave for a few minutes. [5] (ii) Identify the organic functional group present in each of FA 5, FA 6 and FA 7. FA 5 contains the functional group ................................... . FA 6 contains the functional group ................................... . FA 7 contains the functional group ................................... . [3] (b) FA 8 contains one anion and one cation from those listed in the Qualitative Analysis Notes. (i) In a hard‑glass test‑tube heat a spatula measure of FA 8 gently at first and then more strongly. Record all your observations. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [1] (ii) Describe tests that will allow you to identify the cation in FA 8. Carry out these tests and record the tests and your observations in the space below. [3] (iii) Give the formula of the cation present in FA 8. ................................................ [1] [Total: 13] Qualitative Analysis Notes 1 Reactions of aqueous cations reaction with ion NaOH(aq) NH3(aq) aluminium, white ppt. white ppt. Al 3+(aq) soluble in excess insoluble in excess ammonium, no ppt. – NH4+(aq) ammonia produced on heating barium, faint white ppt. is nearly always no ppt. Ba2+(aq) observed unless reagents are pure calcium, white ppt. with high [Ca2+(aq)] no ppt. Ca2+(aq) chromium(III), grey-green ppt. grey-green ppt. Cr3+(aq) soluble in excess insoluble in excess copper(II), pale blue ppt. blue ppt. soluble in excess Cu2+(aq) insoluble in excess giving dark blue solution green ppt. turning brown on contact green ppt. turning brown on contact iron(II), with air with air Fe2+(aq) insoluble in excess insoluble in excess iron(III), red-brown ppt. red-brown ppt. Fe3+(aq) insoluble in excess insoluble in excess magnesium, white ppt. white ppt. Mg2+(aq) insoluble in excess insoluble in excess off-white ppt. rapidly turning brown off-white ppt. rapidly turning brown manganese(II), on contact with air on contact with air Mn2+(aq) insoluble in excess insoluble in excess zinc, white ppt. white ppt. Zn2+(aq) soluble in excess soluble in excess

Mark scheme: 3(a)(i) Award one mark for every two correct observations (*) as shown in table below 5 test FA 5 FA 6 FA 7 + Mg Fizz / bubbling / effervescence * Gas / hydrogen pops with lighted splint * No visible reaction / no change * No visible reaction / no change * + Tollens’ Silver mirror / black / dark grey solid * Silver mirror / black / dark grey solid * No visible reaction / no change * + MnO4‒/H+ (Purple) decolourised * (Purple) decolourised * No visible reaction / no change / remains purple * 3(a)(ii) FA 5 = (carboxylic) acid / COOH / RCOOH 1 FA 6 = aldehyde / CHO / RCHO 1 FA 7 = ketone / RC=OR / RCOR / RCOR/ 1 3(b)(i) Steam / condensation / water vapour and white residue / leaves white solid / solid remains white 1 3(b)(ii) Chooses (aqueous) NaOH and (aqueous) NH3 and indicates use of a solution of FA 8 / FA 8(aq) NaOH white ppt * insol in excess* NH3 white ppt* insol in excess* 1 Question Answer Marks 3(b)(iii) Mg2+ Do not allow magnesium / Mg 1

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