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CIE Nov 2015 v3 Paper 3 Q1
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![The formula of hydrated copper(II) sulfate is CuSO4.xH2O where x is the number of moles of water of crystallisation in one mole of salt. You will determine the value of x by titration. PMT When aqueous copper(II) ions react with aqueous iodide ions, I, iodine is produced. 2Cu2+(aq) + 4I(aq)I2(aq) + 2CuI(s) The amount of iodine, I2, produced can be found by titrating it with aqueous thiosulfate ions, S2O3 2. 2S2O3 2(aq) + I2(aq)S4O6 2(aq) + 2I(aq) FA 1 is aqueous CuSO4.xH2O containing 26.2 g dm3. FA 2 is 0.100 mol dm3 sodium thiosulfate, Na2S2O3. FA 3 is aqueous potassium iodide, KI. starch indicator (a) MethodPipette 25.0 cm3 of FA 1 into a conical flask. Use the measuring cylinder to add 15 cm3 of FA 3, an excess of KI, to the conical flask. The solution will turn brown because iodine is formed. Fill the burette with FA 2. Add FA 2 from the burette until the colour of the mixture changes to pale brown. Add 10 drops of starch indicator. The mixture will turn blue-black. Continue adding FA 2 from the burette until the dark colour suddenly disappears to leave an off-white solid. This is the end point of the titration. Carry out a rough titration and record your burette readings in the space below. The rough titre iscm3. Carry out as many accurate titrations as you think necessary to obtain consistent results. Make certain any recorded results show the precision of your practical work. Record in a suitable form below, all your burette readings and the volume of FA 2 added in each accurate titration. II III IV VI VII [7]UCLES 2015 9701/33/O/N/15<br />
(b) From your accurate titration results, obtain a suitable value to be used in your calculations. Show clearly how you have obtained this value. PMT 25.0 cm3 of FA 1 requiredcm3 of FA 2. [1] (c) Calculations Show your working and appropriate signi cantgures in each step of your calculations. (i) Calculate the number of moles of thiosulfate ions present in the volume of FA 2 you have calculated in (b). (ii) Use your answer to (i), and the equations for the reactions involved, to deduce the number of moles of Cu2+ present in 25.0 cm3 of FA 1. moles of S2O3 2 =mol 2Cu2+(aq) + 4I(aq)I2(aq) + 2CuI(s) 2S2O3 2(aq) + I2(aq)S4O6 2 (aq) + 2I(aq) (iii) Use your answer to (ii) and the mass of CuSO4.xH2O present in the solution, to calculate the relative molecular mass, Mr, of CuSO4.xH2O. moles of Cu2+ =mol (iv) Determine the value of x. (Use data from the Periodic Table on page 12.) Mr of CuSO4.xH2O =UCLES 2015 9701/33/O/N/15 x =[6] [Total: 14] [Turn over<br />](https://learnah.org/wp-content/uploads/2018/08/November_2015_v3_QP_-_Paper_3_CIE_Chemistry_A-level_1-0.jpg)
![Page 2 Mark Scheme Syllabus Paper Cambridge International AS/A LevelOctober/November 2015 9701 33 Question (a) Indicative material Mark Total I Initial and final readings and titre recorded for rough titre and accurate titre details tabulated (minimum 22 boxes) II Headings and units correct for accurate titrations Headings: initial / final (burette) reading / volume or reading / volume at start / finish and titre or volume / FA 2 and added / used / titrated [not difference or total] and Units: (cm3) or / cm3 or in cm3 [or cm3 by every entry] 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) there is only one accurate titration IV Has two uncorrected, accurate titres within 0.1 cm3 Do not award this mark if having performed two titres within 0.1 cm3 a further titration is performed which is more than 0.10 cm3 from the closer of the initial two titres, unless any further titrations, within 0.1 cm3 of any other titration have also been carried out. Do not award the mark if any accurate burette readings (apart from initial 0) are given to zero dp. two (or more) accurate identical titres then two (or more) accurate titres within 0.05 cm3, then Examiner rounds any burette readings to the nearest 0.05 cm3, checks subtractions and then selects the best titres using the hierarchy: (cid:127) (cid:127) two (or more) accurate titres within 0.10 cm3, etc. These best titres should be used to calculate the mean corrected titre to the nearest 0.01 cm3. Examiner calculates the difference () between the mean titres obtained by the candidate and the Supervisor. Accuracy marks awarded as shown. Award V, VI and VII if0.30 cm3 Award V and VI if 0.30 <0.50 cm3 Award V if 0.50 <0.80 cm3. Spread penalty: if the two best titres used by the Examiner were0.5 cm3 apart, cancel one accuracy mark. 1 1 1 [7]Cambridge International Examinations 2015<br />
Page 3 Mark Scheme Syllabus Paper Cambridge International AS/A LevelOctober/November 2015 9701 33 Question (b) Indicative material Mark Total Candidate must average two (or more) accurate titres where the total spread is0.20 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 decimal places rounded to the nearest 0.01. 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. Note: the candidates 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. (c) (i) Correctly calculates moles = (b)0.1 / 1000 and answer to 3 or 4 significant figures (ii) Same as (c)(i) (iii) Shows correct use of either factor of 40 (1000 / 25) or mole ratio Answer consistent with data shown to 3 or 4 sf Mr = 26.2 /( (c)(ii)40) (iv) x = (Mr159.6) / 18 Answer to nearest positive integer with use of either 159.6 or 18 and some use of experimental data. 1 1 [1] [6] Total [14] Qn 1Cambridge International Examinations 2015<br />](https://learnah.org/wp-content/uploads/2018/08/November_2015_v3_MS_-_Paper_3_CIE_Chemistry_A-level_1-0.jpg)
