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CIE Nov 2014 v6 Paper 3 Q1
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![You are to determine the concentration of a solution of sodium thiosulfate, Na2S2O3. PMT To do this you willrst produce a known amount of iodine by reacting iodate(V) ions, IO3 excess of iodide ions, I. The equation for this reaction is below. , with an IO3+ 5I + 6H+3I2 + 3H2O The amount of iodine produced in this reaction can be found by titrating with thiosulfate ions. The equation for this reaction is below. I2 + 2S2O3 22I + S4O6 2 FB 1 is aqueous sodium thiosulfate, Na2S2O3. FB 2 is aqueous potassium iodate(V) containing 3.60 g dm3 KIO3. FB 3 is sulfuric acid, H2SO4. FB 4 is aqueous potassium iodide, KI. starch indicator (a) MethodFill a burette with FB 1. Pipette 25.0 cm3 of FB 2 into the conical flask. Use the measuring cylinder to add 25 cm3 of FB 3 into the conical flask. Use the measuring cylinder to add 10 cm3 of FB 4 into the conical flask. Brown iodine solution is produced. Add FB 1 from the burette until most of the iodine has been removed and the solution in the conical flask is yellow. Add 10 drops of starch indicator to the contents of the conical flask. The solution will turn blue-black. Continue adding FB 1, from the burette, until the blue-black colour just disappears. 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 of your burette readings and the volume of FB 1 added in each accurate titration. II III IV VI [6]UCLES 2014 9701/36/O/N/14<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 FB 2 requiredcm3 of FB 1. [1] (c) Calculations Show your working and appropriate signi cantgures in thenal answer to each step of your calculations. (i) Calculate the number of moles of KIO3 present in 25.0 cm3 of FB 2. [Ar: O, 16.0; K, 39.1; I, 126.9] (ii) The equations for the production of iodine and its titration with thiosulfate are shown below. moles of KIO3 =mol IO3+ 5I + 6H+3I2 + 3H2O I2 + 2S2O3 22I + S4O6 2 Use these equations to calculate the number of moles of thiosulfate present in the volume of FB 1 you calculated in (b). (iii) Calculate the concentration, in mol dm3, of sodium thiosulfate in FB 1. moles of S2O3 2 =mol concentration =mol dm3 [5] [Total: 12]UCLES 2014 9701/36/O/N/14 [Turn over<br />](https://learnah.org/wp-content/uploads/2018/08/November_2014_v6_QP_-_Paper_3_CIE_Chemistry_A-level_1-0.jpg)
![Page 2 Mark Scheme Syllabus Paper Cambridge International AS/A LevelOctober/November 2014 9701 36 Question (a) Indicative material The following data is showntwo burette readings for the rough titrationtitre for rough titrationinitial and final burette readings for two (or more) accurate titrations (Minimum of 22 boxes) Mark Total Correct headings and units for accurate titrations. Headings should match readingsFinal / end and (burette) reading / volumeTitre or volume / Initial / start and (burette) reading / volume (not V or vol) FB 1 and used / added (not difference, total or change) Unit: / cm3 or (cm3) or in cm3 or cm3 for each reading. 3 II All accurate burette readings are to nearest 0.05 cm . applies to the burette readings The need to record to 0.05 cm 3 and not to the recorded titres but it does apply to 0.00 cm . 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) 3 III Has two uncorrected, accurate titres within 0.1 cm Do not consider the rough even if ticked. 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 this mark if any accurate burette readings (apart from initial 0) are given to zero dp. 3 Round any burette readings to the nearest 0.05 cm . Examiner selects the best titres using the hierarchy:These best titres should be used to calculate the mean corrected 3 titre to the nearest 0.01 cm . two (or more) accurate identical titres (ignoring rough), then 3 two (or more) accurate titres within 0.05 cm , then 3 two (or more) accurate titres within 0.10 cm etc 3 Award IV, V and VI if<0.2 cm 3 Award IV and V if>0.2 but < 0.3 cm 3 3 Award IV if>0.3 cm . but <0.4 cm Spread penalty: if the two best (corrected) titres used by the 3 examiner were >0.5 cm apart, cancel one Q mark. 1 1 1Cambridge International Examinations 2014 [6]<br />
Page 3 Mark Scheme Syllabus Paper Cambridge International AS/A LevelOctober/November 2014 9701 36 (b) Calculation of mean Candidate must average two (or more) accurate titres with total spread of no more than 0.2 cm 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. Example: 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: any selected titre is not within 0.20 cm3 of any other selected titre; the rough titre was used to calculate the mean; the 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 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) Mr KIO3 = 214 Moles dm3 3.60 40 214 = 4.205 / 4.206 / 4.21 / 4.21104 (ii) Moles S2O3 2 = (i)6 = (2.52103 ) (iii) (ii) 1000 (b) from vol Answers given to 3 or 4 sf 1 1 [1] [5] [Total: 12] (a) Round times to nearest second. 3 3 Supervisor calculates time with 10 cm / time with 20 cm (to 1 dp) and awards 3 marks if within 1.9 to 2.1 awards 2 marks if within 1.8 to 2.2 (but not within 1.9 to 2.1) awards 1 mark if within 1.6 and 2.4 (but not within 1.8 to 2.2) (b) 3 3 additional volumes chosen with intervals not less than 2 cm . 3 These must include 1 of < 10 cm and have and 1 of > 10 cm 3 none < 4 cm . 1 1 1 [3]Cambridge International Examinations 2014<br />](https://learnah.org/wp-content/uploads/2018/08/November_2014_v6_MS_-_Paper_3_CIE_Chemistry_A-level_1-0.jpg)
