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CIE Jun 2015 v2 Paper 3 Q1
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![In this question you will determine the percentage purity of a sample of contaminated sodium carbonate. FB 1 is a solution of the contaminated sodium carbonate. You willrst dilute FB 1 and then titrate the diluted solution using hydrochloric acid. 2HCl (aq) + Na2CO3(aq)2NaCl (aq) + H2O(l) + CO2(g) FB 1 was prepared by dissolving 125 g of contaminated sodium carbonate, Na2CO3, in distilled water and making the solution up to 1 dm3. FB 2 is 0.100 mol dm3 hydrochloric acid, HCl. methyl orange indicator (a) Method DilutionFill the burette with FB 1. Run between 13.00 and 13.50 cm3 of FB 1 into the 250 cm3 volumetric (graduated) flask. Record the volume in the space below. Fill the volumetric flask to the line with distilled water. Stopper the flask and shake it to ensure thorough mixing. Label this flask FB 3. volume of FB 1 =cm3 TitrationRinse the burette thoroughly with distilled water and then with a little of solution FB 2. Fill the burette with FB 2. Use the pipette to transfer 25.0 cm3 of FB 3 into a conical flask. Add 5-10 drops of methyl orange indicator. Perform a rough titration and record your burette readings in the space below. The rough titre iscm3. Do 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 2 added in each accurate titration. II III IV VIUCLES 2015 9701/32/M/J/15 VII VIII [8]<br />
(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 FB 3 requiredcm3 of FB 2. [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 hydrochloric acid present in the volume of FB 2 calculated in (b). (ii) Calculate the number of moles of sodium carbonate present in 25.0 cm3 of FB 3. moles of HCl =mol (iii) Calculate the concentration, in mol dm3, of sodium carbonate in FB 3. moles of Na2CO3 =mol (iv) Calculate the concentration, in mol dm3, of sodium carbonate in FB 1. concentration of Na2CO3 in FB 3 =mol dm3 concentration of Na2CO3 in FB 1 =mol dm3 (v) Calculate the percentage purity by mass of the sodium carbonate in the contaminated sample used to prepare solution FB 1. (Ar: C, 12.0; O, 16.0; Na, 23.0) percentage purity by mass =% [5] II III IVUCLES 2015 9701/32/M/J/15 [Total: 14] [Turn over<br />](https://learnah.org/wp-content/uploads/2018/08/June_2015_v2_QP_-_Paper_3_CIE_Chemistry_A-level_1-0.jpg)
![Page 2 Question Cambridge International AS/A LevelMay/June 2015 9701 32 Mark Scheme Syllabus Paper Indicative material Mark Total (a) I Follows instructionsadds between 13.00 and 13.50 cm3 II Initial and final burette readings and titre unambiguously recorded in rough and accurate titrations. III Headings and units correct for accurate titration and headings match readings. Headings: initial / final (burette) reading / volume or Reading / volume at start / finish and Volume / difference] and Units: (cm3) or / cm3 or in cm3 or cm3 by every entry FB 1 added / used or titre [not IV All accurate burette readings (initial and final) recorded to 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) V 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 that is more than 0.1 cm3 from the closer of the two initial titres unless further titrations within 0.1 cm3 of any other has also been carried out. Do not award the mark if any accurate burette readings (apart from initial 0) are given to zero dp. Examiner rounds all burette readings to the nearest 0.05 cm3 and checks subtractions. The best titres should be selected using the hierarchy: two (or more) identical, then two (or more) within 0.05 cm3 , then two (or more) within 0.1 cm3, etc. Examiner to calculate, correct to 2 dp, supervisor titre supervisor volume diluted and candidate titre cand vol diluted and find the difference, . Award VI, VII and VIII if0.02 Award VI and VII if 0.200.04 Award VI if 0.400.06 Spread penalty: if the two best titres used by the examiner are more than 0.50 cm3 apart cancel one of the Q marks. 1 1 1 [8]Cambridge International Examinations 2015<br />
Page 3 (b) Question Cambridge International AS/A LevelMay/June 2015 9701 32 Mark Scheme Syllabus Paper Calculation of mean 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 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. [1] Indicative material Mark Total 1 (c) (i)(ii) I Uses the expression 0.100 (b) 1000 in step (i) and use ofin (ii) (iii) II Correctly calculates (ii) / 0.025 (iv) (v) III Correctly calculates (iii)(250 / volume diluted) IV Correctly calculates (iv)(106 / 125)100 V Answers to (i) to (iv) given to 3 or 4 sf Qn 1 Total [5] [14]Cambridge International Examinations 2015<br />](https://learnah.org/wp-content/uploads/2018/08/June_2015_v2_MS_-_Paper_3_CIE_Chemistry_A-level_1-0.jpg)
