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CIE Jun 2017 v2 Paper 3 Q1
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![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. PMT HA(aq) + NaOH(aq) NaA(aq) + H2O(l) FB 1 is a solution of organic acid, HA, containing 12.60 g dm3. FB 2 is 0.100 mol dm3 sodium hydroxide, NaOH. thymol blue indicator (a) MethodFill 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 iscm3.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. II III IV 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 requiredcm3 of FB 1. [1]UCLES 2017 9701/32/M/J/17<br />
PMT (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 dm3. concentration of HA in FB 1 =mol dm3 (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. (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. Mr of HA =anion, A =[4]UCLES 2017 9701/32/M/J/17 [Turn over<br />
PMT (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]UCLES 2017 9701/32/M/J/17<br />](https://learnah.org/wp-content/uploads/2018/08/June_2017_v2_QP_-_Paper_3_CIE_Chemistry_A-level_1-0.jpg)
![9701/32 Question 1(a) Cambridge International AS/A LevelMark Scheme PUBLISHED Answer I Initial and final burette readings and volume added recorded for rough titre and accurate titre details tabulated. [minimum 22 boxes with relevant information] 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] 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) IV The final accurate titre recorded is within 0.10 cm3 of any other accurate titre. PMT May/June 2017 Marks 1UCLES 2017 Page 2 of 9<br />
9701/32 Question 1(a) 1(b) 1(c)(i)UCLES 2017 Cambridge International AS/A LevelMark Scheme PUBLISHED Answer 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 candidates titre is compared to the supervisors titre andcalculated. V, VI and VII Award V, VI and VII for0.20 cm3 Award V and VI for 0.20 cm3 <0.30 cm3 Award V for 0.30 cm3 <0.50 cm3Candidate must average two (or more) titres where the total spread is0.20 cm3. Check mean titre is correctly calculated from clearly selected values (ticks or working).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: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. Correctly calculates 0.100 x 25 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. = 2.5(0)103 1000 Page 3 of 9 PMT May/June 2017 Marks<br />
9701/32 Question 1(c)(ii) 1(c)(iii) 1(c)(iv) 1(d)(i) 1(d)(ii) Cambridge International AS/A LevelMark Scheme PUBLISHED Answer Correctly calculates 0.0025 x 1000 to 3 or 4 sf ) Correct expression 12.6(ii) Anion in FB 1 = CHCl2COO (allow ecf: for candidates answer to (iii)) CH3COO :77 CCl3COO :146.5 CH2ClCOO : 77.5111.5 CHCl2COO : 112146 Conc NaOH lower = > titre smaller = > smaller Mr 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 Question 2(a) 2(b)(i) Answer I Unambiguous headings and correct units tabulated for all 6 thermometer readings, mean temps, and Ts 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 Award III if candidate Ts within 1.5 C Award III and IV if candidate Ts within 1.0 C Correctly calculates n acid = 0.05(0) mol and n NaOH = 0.045 molUCLES 2017 Page 4 of 9 PMT May/June 2017 Marks 1 1 1 1 1 Total: 14 Marks 1 1 1 1 1<br />](https://learnah.org/wp-content/uploads/2018/08/June_2017_v2_MS_-_Paper_3_CIE_Chemistry_A-level_1-0.jpg)
