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CIE Nov 2013 v5 Paper 3 Q1
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![Examiners For Use You are provided with a solution of an organic acid which is known to be one of the following. methanoic acid, HCOOH propanoic acid, C2H5COOH pent-2-enoic acid, CH3CH2CH=CHCOOH The solution was made by dissolving 1.85 g of acid in 250 cm3 of solution. You are to suggest the identity of the acid bynding its relative molecular mass, Mr, using a titration method. FA 1 is the solution of the unknown organic acid. FA 2 is 0.100 mol dm3 sodium hydroxide, NaOH. phenolphthalein indicator (a) MethodFill the burette with FA 2. Pipette 25.0 cm3 of FA 1 into a conical flask. Titrate FA 1 with FA 2 using phenolphthalein as indicator. 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 2 added in each accurate titration. II III IV VI VII [7]UCLES 2013 9701/35/O/N/13<br />
Examiners For Use (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 1 requiredcm3 of FA 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 sodium hydroxide in the volume of FA 2 you have calculated in (b). (ii) One mole of any of the organic acids reacts with one mole of sodium hydroxide. Calculate the concentration, in mol dm3, of the acid in FA 1. moles of NaOH =mol (iii) Calculate the concentration, in g dm3, of the acid used to make solution FA 1. concentration of the acid in FA 1 =mol dm3 concentration of the acid in FA 1 =g dm3 (iv) Using your answers to (ii) and (iii), calculate the relative molecular mass, Mr, of the acid in FA 1.UCLES 2013 9701/35/O/N/13 [Turn over Mr of the acid = <br />
Examiners For Use (v) Suggest which of the acids, methanoic, propanoic or pent-2-enoic acid, is present in FA 1. Acid present is. (vi) Suggest a test that could be carried out to distinguish pent-2-enoic acid from methanoic acid and propanoic acid. Give the expected result of your test. testexpected result[5] [Total: 13]UCLES 2013 9701/35/O/N/13<br />](https://learnah.org/wp-content/uploads/2018/08/November_2013_v5_QP_-_Paper_3_CIE_Chemistry_A-level_1-0.jpg)
![Page 2 Mark Scheme Syllabus Paper GCE A LEVELOctober/November 2013 9701 35 Question Sections Indicative material Mark (a) PDO Volume given for rough titre Layout and accurate burette readings tabulated. (min of 22 box) MMO II Initial and final burette readings recorded for Collection rough titre PDO Recording MMO Decisions and volume of FA 2 added recorded for each accurate titre and acceptable headings and units in the accurate section. Acceptable headings are initial (burette) reading / initial volume / first reading / start reading Final (burette) reading / final volume / 2 reading / end reading Titre / volume used / volume added / FA 2 nd added.(not difference or change in) Acceptable units are / cm 3 / in cm 3 / (cm ) / cm by each reading. III All accurate burette readings recorded to nearest 0.05 cm Do not award this mark if: 50 (.00) is used as an initial burette readingMore than one final burette reading is 50 (.00)Any burette reading is greater than 50. (00) IV Has two uncorrected, accurate titres within 0.1 cm Do not include a reading labelled rough. Do not award this mark if, having performed 2 titres , a further titration is carried out which within 0.1 cm is > 0.1 cm from the closer of the 2 initial titres unless further titrations, within 0.1 of any others, have also been carried out. Do not award the mark if any accurate burette readings (apart from initial zero) are given as integer.Cambridge International Examinations 2013<br />
Page 3 Mark Scheme Syllabus Paper GCE A LEVELOctober/November 2013 9701 35 MMO V, VI and VII Quality All burette readings should be rounded to the nearest 0.05 cm . Subtractions should be checked. Examiner then selects the best titre using the hierarchy: two identical, titres within 0.05 cm etc. These best titres should be used to calculate the mean titre to the nearest 0.01 cm Award V, VI and VII for a difference from Supervisor0.2 cm Award V and VI for a difference from Supervisor 0.2 <0.3 cm Award V for a difference from Supervisor 0.3 <to 0.5 cm Spread penalty: if the two best titres used by the Examiner are0.50 cm apart, cancel 1 Q mark. [7] (b) ACE Candidate calculates the mean correctly. Interpretation Candidate must take the average of two (or more) titres where the total spread is0.2 cm Working must be shown or ticks must be placed next to the accurate titres selected. The mean should normally be shown to 2 dp, rounded to the nearest 0.01 cm . Example 26.667 must be rounded to 26.67 and not 26.65 and 26.675 must be rounded to 26.68 and not 26.70. Two special cases where the mean may not be to 2 dp: Allow mean to 3dp 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 incorrectshould be 26.05) Do not award this mark if: The rough titre was used to calculate the mean. The candidate performed only one 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 correct even if it is different from the mean calculated by the Examiner for the purpose of assessing accuracy. [1]Cambridge International Examinations 2013<br />
Page 4 Mark Scheme Syllabus Paper GCE A LEVELOctober/November 2013 9701 35 (c) ACE I Correctly calculates moles of NaOH = (b)0.1 in (i) Interpretation 1000 and (i)1000 in (ii) 25.0 II 1.854 = 7.40 (g dm and (iii) in (iv) (ii) PDO Display ACE Conclusions MMO Decisions III All answers to 3 or 4 sf (minimum of 3 answers) IV Acid with nearest Mr. Conclusion must correspond to Mr. V Test(aqueous) bromine / acidified KMnO4 alkaline KMnO4. Expected resultdecolorises / (goes) colourless) / decolorised / turns green. [5] [Total: 13]Cambridge International Examinations 2013<br />](https://learnah.org/wp-content/uploads/2018/08/November_2013_v5_MS_-_Paper_3_CIE_Chemistry_A-level_1-0.jpg)
