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CIE Jun 2015 v3 Paper 3 Q1
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![HA is an organic acid. Solution FA 1 was prepared by dissolving 13.1 g of solid HA in each dm3 of solution. You are to determine the relative molecular mass, Mr, of HA by titration with aqueous sodium hydroxide. The equation for the reaction between HA and sodium hydroxide is given below. HA(aq) + NaOH(aq)NaA(aq) + H2O(l) FA 1 is a solution containing 13.1 g dm3 of organic acid, HA. FA 2 is 0.100 mol dm3 sodium hydroxide, NaOH. bromothymol blue indicator (a) MethodFill the burette with FA 1. Use the pipette to transfer 25.0 cm3 of FA 2 into a conical flask. Add a few drops of bromothymol blue indicator. This indicator is blue in alkali and yellow in acid 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 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 FA 1 added in each accurate titration. II III IV VI VII [7] (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 FA 2 requiredcm3 of FA 1. [1]UCLES 2015 9701/33/M/J/15<br />
(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 sodium hydroxide present in the volume of FA 2 pipetted into the conicalask. (ii) Use your answer to (c)(i) and the equation on page 2 to determine the number of moles of organic acid, HA, used to neutralise 25.0 cm3 of FA 2. moles of NaOH =mol (iii) Use your answers to (b) and (c)(ii) to calculate the number of moles of HA in 1 dm3 of FA 1. moles of HA =mol (iv) Calculate the relative molecular mass, Mr, of the organic acid, HA. moles of HA in 1 dm3 of FA 1 =mol Mr of HA =[4] (d) A student carrying out this method correctly with the same concentration of reactants obtained a titre of 28.30 cm3. Would this give a larger or smaller value of Mr than yours? Explain your answer.[1]UCLES 2015 9701/33/M/J/15 [Total: 13] [Turn over<br />](https://learnah.org/wp-content/uploads/2018/08/June_2015_v3_QP_-_Paper_3_CIE_Chemistry_A-level_1-0.jpg)
![Page 2 Cambridge International AS/A LevelMay/June 2015 9701 33 Mark Scheme Syllabus Paper Question Indicative material Mark Total (a) I The following data must be shownburette readings and titre for rough titration 22 box showing both accurate burette readings II Headings and units correct for accurate titration table and headings match readings.initial / start (burette) reading / volume + unit final / end (burette) reading / volume + unit titre or volume / FA 1 used / added (not difference or total) + unit Units: (cm3) or / cm3 or in cm3 or cm3 by every entry III All accurate burette readings to 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 Two accurate titres within 0.10 cm3. Do not award if 3rd titre > 0.10 cm3 away from either previous titre unless a further titration is also carried out which is within 0.1 cm3 of any other. Do not award the mark if any accurate burette readings (apart from initial 0) are given to zero dp. Examiner checks and corrects titre subtractions where necessary. Examiner selects the best mean titre using a hierarchy: two identical titres within 0.05 cm3, two or more titres within 0.10 cm3 etc. Examiner subtracts (corrected) candidates titre from Supervisors titre. Award V, VI and VII if< 0.20 cm3 Award V and VI if 0.20 <<0.30 cm3 Award V if 0.30 << 0.50 cm3 Spread penalty: if the two best titres are0.50 cm3 apart, cancel one of the Q marks 1 1 1 [7]Cambridge International Examinations 2015<br />
Cambridge International AS/A LevelMay/June 2015 9701 33 Mark Scheme Syllabus Paper Page 3 (b) [1] [4] [1] [13] 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 titres selected. The mean should normally be quoted to 2 dp 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)(ii) Correctly calculates 0.1 25 1000 = 2.5(0)10 3 and (iii) (iv) (d) (ii) = (i) Correctly calculates )( iic 1000 )( b Correct expression 13.1 ( iii or correct answer if no working All answers to (iii) and (iv) are given to 3 or 4 sf (minimum of 3 parts attempted) If candidate titre < 28.30 cm3, students Mr larger as smaller concentration / molarity / moles in 1 dm3 of HA / FA 1 than candidate (for same mass) If candidate titre > 28.30cm3, reverse argument to above If candidate titre = 28.30cm3 no change / no difference in Mr as concentration same as HA / FA 1 (owtte) Qn 1Cambridge International Examinations 2015<br />](https://learnah.org/wp-content/uploads/2018/08/June_2015_v3_MS_-_Paper_3_CIE_Chemistry_A-level_1-0.jpg)
