Cambridge A Level Chemistry 9701 — 2017 May/June Paper 3 · Variant 2

1 HA is an organic acid where A– is the anion. You will determine the relative formula mass, Mr, of HA by titration with sodium hydroxide of known concentration and so identify the anion, A–. The equation for the reaction is shown. HA(aq) + NaOH(aq) NaA(aq) + H2O(l) FB 1 is a solution of organic acid, HA, containing 12.60 g dm–3. FB 2 is 0.100 mol dm–3 sodium hydroxide, NaOH. thymol blue indicator (a) Method ● Fill the burette with FB 1. ● Pipette 25.0 cm3 of FB 2 into a conical flask. ● Add approximately 10 drops of thymol blue indicator. This indicator is blue in alkaline solutions and yellow in acidic solutions. ● 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 certain that 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 1 added in each accurate titration. I II III IV V VI VII [7] (b) From your accurate titration results, obtain a suitable value for the volume of FB 1 to be used in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FB 2 required ............................ cm3 of FB 1. [1] (c) Calculations Show your working and appropriate significant figures in the final answer to each step of your calculations. (i) Calculate the number of moles of sodium hydroxide present in 25.0 cm3 of FB 2 pipetted into the conical flask. moles of NaOH = ............................ mol (ii) Use your answers to (b) and (c)(i) to determine the concentration of the organic acid HA, in FB 1, in mol dm–3. concentration of HA in FB 1 = ............................ mol dm–3 (iii) Use your answer to (ii) and the information given on page 2 to determine the relative formula mass, Mr, of the organic acid, HA. Mr of HA = ............................ (iv) The organic acid was known to have one of the following structural formulae. CH3COOH CH2Cl COOH CHCl 2COOH CCl 3COOH Use your answer to (iii) and the Periodic Table on page 16 to identify the anion, A–. anion, A– = ............................ [4] (d) A student carried out the same procedure accurately but was supplied with a solution of less concentrated sodium hydroxide by mistake. (i) What effect would this have on the calculated value of the relative formula mass, Mr? Explain your answer. ............................................................................................................................................. ............................................................................................................................................. (ii) Explain how this would affect the identification of the acid. ............................................................................................................................................. ............................................................................................................................................. [2] [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 volume of FB 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 / FB 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 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) 1 IV The final accurate titre recorded is within 0.10 cm3 of any other accurate titre. 1 Question Answer Marks 1(a) For assessment of accuracy (Q) marks, each Examiner should round any accurate burette readings to the nearest 0.05 cm3, check subtractions and then select the “best” titres for supervisor and candidate using the hierarchy: two identical; titres within 0.05 cm3; titres within 0.1 cm3; etc. These best titres should be used to calculate the mean titre, expressed to the nearest 0.01 cm3. The candidate’s titre is compared to the supervisor’s titre and δ calculated. V, VI and VII Award V, VI and VII for δ ⩽ 0.20 cm3 Award V and VI for 0.20 cm3 < δ ⩽ 0.30 cm3 Award V for 0.30 cm3 < δ ⩽ 0.50 cm3 3 1(b) Check mean titre is correctly calculated from clearly selected values (ticks or working). • Candidate must average two (or more) 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. • The mean should normally be quoted to 2 dp rounded to the nearest 0.01. [e.g. 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: • the rough titre was used to calculate the mean; • candidate carried out only 1 accurate titration; • burette readings were incorrectly subtracted to obtain any of the accurate titre values; • all burette readings (resulting in titre values used in calculation of mean) are integers. 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 1(c)(i) Correctly calculates 0.100 x 25 1000 = 2.5(0) × 10–3 1 Question Answer Marks 1(c)(ii) Correctly calculates ( ) 0.0025 x 1000 b to 3 or 4 sf 1 1(c)(iii) Correct expression 12.6 ÷ (ii) 1 1(c)(iv) Anion in FB 1 = CHCl2COO– (allow ecf: for candidate’s answer to (iii)) CH3COO– : ⩽ 77 CH2ClCOO– : 77.5 – 111.5 CHCl2COO– : 112 – 146 CCl3COO– : ⩾ 146.5 1 1(d)(i) Conc NaOH lower = > titre smaller = > smaller Mr 1 1(d)(ii) No effect on identification unless closer to smaller mass acid or (different Mr may lead to the) identification of a different acid with matching / close to Mr 1 Total: 14

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2 You are to determine the enthalpy change of neutralisation for a different acid from that used in Question 1. The acid is represented by HB where B– represents the anion. HB(aq) + NaOH(aq) NaB(aq) + H2O(l) FB 3 is 2.00 mol dm–3 acid, HB. FB 4 is 72.00 g dm–3 sodium hydroxide, NaOH. (a) Method Read through the method before starting your practical work and prepare a table below for recording your results. Experiment 1 ● Place the plastic cup in the 250 cm3 beaker. ● Pour 25 cm3 of FB 3 into the larger measuring cylinder. ● Measure and record the temperature of FB 3. ● Rinse and dry the thermometer. ● Use the smaller measuring cylinder to transfer 25 cm3 of FB 4 into the plastic cup. ● Measure and record the temperature of FB 4. ● Add the 25 cm3 of FB 3 to FB 4 in the plastic cup and stir the mixture. Measure and record the highest temperature reached. ● Calculate and record the average initial temperature of the solutions. ● Calculate and record the temperature rise. ● Empty the plastic cup, rinse it with water and shake it to remove excess water. Experiment 2 ● Repeat the method given for Experiment 1 using 50 cm3 of each solution. ● Use the larger measuring cylinder for FB 3 and the smaller measuring cylinder for FB 4. Results I II III IV [4] (b) Calculations Show your working and appropriate significant figures in the final answer to each step of your calculations. (i) Show by calculation that in Experiment 1, the number of moles of acid was in excess of the number of moles of sodium hydroxide. (ii) Calculate the heat energy evolved in Experiment 1. [Assume that 4.2 J of heat energy changes the temperature of 1.0 cm3 of solution by 1.0 °C.] heat energy evolved = ............................ J (iii) Calculate the enthalpy change, in kJ mol–1, for Experiment 1. enthalpy change = ...... ............................. kJ mol–1 (sign) (value) (iv) Calculate the number of moles of sodium hydroxide neutralised in Experiment 2. moles of NaOH = ............................ mol (v) Calculate the enthalpy change, in kJ mol–1, for Experiment 2. enthalpy change = ...... ............................. kJ mol–1 (sign) (value) [5] (c) (i) The accuracy of the larger measuring cylinder is ± 0.5 cm3. The accuracy of the smaller measuring cylinder is ± 0.25 cm3. Calculate the maximum percentage error in the measurement of the volume of FB 3 used in Experiment 2 and the measurement of the volume of FB 4 used in Experiment 2. Show your working. maximum % error in volume of FB 3 ............................ maximum % error in volume of FB 4 ............................ (ii) Suggest a change to the method used in (a) that would improve the accuracy of your results. ............................................................................................................................................. ............................................................................................................................................. [3] [Total: 12]

Mark scheme: 2(a) 1 II All thermometer readings recorded to 0.5 °C and ∆Ts correctly calculated and Mean temperatures correctly calculated to nearest .5 °C or to 1 or 2 dp 1 Award III if candidate ∆Ts within 1.5 °C 1 Award III and IV if candidate ∆Ts within 1.0 °C 1 2(b)(i) Correctly calculates n acid = 0.05(0) mol and n NaOH = 0.045 mol 1 Question Answer Marks 2(b)(ii) Correctly calculates 50 × 4.2 × ∆T1 to minimum 2 sf 1 2(b)(iii) Correct expression ( ) 1000 x 0.045 ii 1 2(b)(iv) +(v) 2 × mol NaOH in (i) or 0.09(0) in (iv) and Correctly uses ( ) ( ) ∆ 100 x 4.2 x T 2 1000 x iv in (v) 1 Negative signs shown in (iii) and (v) and final answers to 2–4 sf in (ii), (iii) & (v) 1 2(c)(i) % error in vol of FB 3 = 0.5 x 100 50 = 1.(0)% 1 % error in vol of FB 4 = 2 x 0.25 x 100 25 = 2.(0)% 1 2(c)(ii) Use a burette / pipette for volume measurements / instead of a measuring cylinder or Add a lid to reduce heat loss (by convection) / to reduce convection or Use thermometer reading to 0.2 °C / smaller divisions / calibrations / more sensitive 1 Total 12

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3 Qualitative Analysis At each stage of any test you are to record details of the following. ● colour changes seen ● the formation of any precipitate ● the solubility of such precipitates in an excess of the reagent added Where reagents are selected for use in a test, the name or correct formula of the element or compound must be given. Where gases are released they should be identified by a test, described in the appropriate place in your observations. You should indicate clearly at what stage in a test a change occurs. No additional tests for ions present should be attempted. If any solution is warmed, a boiling tube MUST be used. Rinse and reuse test-tubes and boiling tubes where possible. (a) FB 5 and FB 6 are solutions of acids of equal concentration in mol dm–3. One solution is a weak acid and the other is a strong acid. (i) Devise and carry out a chemical test to find out which of FB 5 and FB 6 is the weak acid. Record your test, observations and conclusion in the space below. (ii) Another acid, FB 7, is a dilute solution of one of hydrochloric, nitric or sulfuric acids. Carry out the tests in the order given in the table below until you are able to identify FB 7. Record your observations. If any test is unnecessary write ‘not needed’. test observations To a 1 cm depth of FB 7 in a test-tube add aqueous silver nitrate. To a 0.5 cm depth of FB 7 in a boiling tube add a 1 cm depth of aqueous sodium hydroxide and a small piece of aluminium foil and warm. To a 1 cm depth of FB 7 in a test-tube add aqueous barium chloride or aqueous barium nitrate. FB 7 is ............................................... acid. [6] (b) FB 8 contains a cation listed in the Qualitative Analysis Notes. FB 9 is a solution of an organic salt. Carry out the following tests and record your observations. test observations (i) To a 1 cm depth of FB 8 in a test-tube add aqueous sodium carbonate. (ii) To a 1 cm depth of FB 8 in a test-tube add a 1 cm depth of aqueous potassium iodide, then add aqueous sodium thiosulfate until in excess. (iii) To a 2 cm depth of FB 8 in a test-tube add a 1 cm depth of concentrated hydrochloric acid (CARE). Keep this solution for test (iv). (iv) To a 2 cm depth of distilled water in a boiling tube add all the contents of the test-tube from test (iii). Keep this solution for tests (v) and (vi). Pour a 1 cm depth of the contents of the boiling tube into three separate test-tubes for use in tests (v) and (vi). One tube is to be used for comparing colours in your observations. (v) To one of the test-tubes add aqueous ammonia until in excess. (vi) To a second test-tube add FB 9 until in excess.

Mark scheme: 3(a)(i) Selects Na2CO3 / Mg 1 Effervescence / bubbling / fizzing greater / faster with FB 6 1 FB 5 is the weak acid (ora) with some evidence 1 3(a)(ii) no reaction / no ppt / no change with Ag+ and ‘not needed’ with Ba2+ (do not allow ‘no change’ unless there is no evidence of ammonia in 2nd test) Effervescence alone is not evidence so would expect ‘no change’. 1 Effervescence / gas / NH3 and turns (damp red) litmus blue 1 FB 7 is nitric acid from some evidence (can be effervescence in (ii)) 1 Question Answer Marks 3(b) (i) – (vi) See below 6 Expected observations test observation mark (i) + Na2CO3 (pale) blue ppt (Allow blue-green / green-blue / turquoise / cyan) 1 (ii) + KI then Yellow-brown / brown (not orange) 1 Na2S2O3 white / off-white ppt and soluble in excess 1 (iii) + c.HCl and (blue) (solution) turns green (shade greener) (iv) + H2O (green) (solution) turns (pale) blue (shade bluer) 1 (v) + NH3 (in excess) forming dark/deep blue solution or solution much darker than (iv) 1 (vi) + edta (solution) more blue / darker blue than (iv) 1 Question Answer Marks 3(vii) FB 8 contains Cu2+/copper(II) 1 3(viii) Cu2+(aq) + CO3 2–(aq) → CuCO3(s) Allow 2Cu2+(aq) + 4I–(aq) → 2CuI(s) + I2(aq or s) Allow Cu2+(aq) + 2OH–(aq) → 2Cu(OH)2(s) 1 Total: 14 Skill Minimum mark allocation Breakdown of marks Question 1 Question 2 Question 3 Total mark Statement Minimum Marks Manipulation, measurement and observation (MMO) 12 marks [17] Successful collection of data and observations C 8 1 9 10 Quality of measurements and observations Q 2 3 2 5 Decisions relating to measurements of observations De 2 1 1 2 Presentation of data and observations (PDO) 6 marks [7] Recording data or observations R 2 1 1 2 Display of calculation and reasoning Di 2 1 2 3 Data layout L 2 1 1 2 Analysis, conclusions and evaluation (ACE) 10 marks [16] Interpretation of data or observations and identifying sources of error I 4 3 5 1 9 Drawing conclusions Con 5 3 3 6 Suggesting improvements Imp 1 1 1 Total 14 12 14 40

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