Edexcel Jun 2010 Paper 2 Q19 (with explained solutions) |

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Edexcel Jun 2010 Paper 2 Q19

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PMT SECTION B Answer ALL the questions. Write your answers in the spaces provided. 19 A student carried out an experiment to determine the concentration of ethanoic acid in a solution of vinegar. solution. flasks.The student used a measuring cylinder to measure out 25.0 cm3 of the vinegarThis solution was then transferred to a 250 cm3 volumetric flask and the liquid level was carefully made up to the mark with distilled water.A pipette was used to transfer 25.0 cm3 portions of the acidic solution to conicalThe solution was then titrated with sodium hydroxide solution, concentration 0.100 mol dm3, using phenolphthalein as the indicator. CH3COOH(aq) + NaOH(aq)CH3COONa(aq) + H2O(l) Results Titration number Burette reading ( nal) / cm3 Burette reading (initial) / cm3 Volume of NaOH used / cm3 28.55 0.00 28.55 28.00 0.05 27.95 40.35 12.30 28.05 28.05 0.05 28.00 (a) In this titration, what is the colour change of the phenolphthalein indicator? (2) Fromto(b) Explain why the mean titre should be based only on titrations 2, 3 and 4. (1)10 *N36390A01024*<br />
PMT (c) Calculate the mean titre in cm3. (1) (d) (i) Using your answer to (c), calculate the number of moles of sodium hydroxide in the mean titre. (ii) Hence state the number of moles of ethanoic acid, CH3COOH, in 25.0 cm3 of the diluted solution used in the titration. (iii) Calculate the concentration of the diluted acid solution in mol dm3. (1) (1) (1) *N36390A01124* 11 Turn over<br />
PMT (1) (1) (2) (iv) Hence calculate the concentration of the ethanoic acid in the original vinegar solution in mol dm3. (v) Use your answer from (d)(iv) to state the concentration of the ethanoic acid in the original vinegar solution in units of g dm3. [The molar mass of the ethanoic acid is 60 g mol1.] (e) Suggest, with a reason, how the students method of preparing the diluted solution could be improved. ImprovementReason12 *N36390A01224*<br />
PMT (f) The burette used in the titration had an uncertainty for each reading of +0.05 cm3. Identify, by letter, which ONE of the following should be regarded as the true value of the titre in titration number 2? (i) X Between 27.90 and 28.00 cm3 Y Between 27.925 and 27.975 cm3 Z Between 27.85 and 28.05 cm3 (1)(ii) Suggest ONE reason why a student may obtain volumes outside the uncertainty of the burette when performing a titration. (1)(Total for Question 19 = 13 marks) *N36390A01324* 13 Turn over<br />

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Section B Question Number 19 (a) Question Number 19 (b) Question Number 19 (c) Acceptable Answers Reject Mark Mark independently From: colourless (1) To: pink / (pale) red (1) If colour change wrong way round max (1) From: clear To: magenta / purple / cerise Acceptable Answers Reject Mark (Titres 2, 3 and 4) are concordant / within 0.2 (cm3) / within 0.1 (cm3) / consistent OR Titre 1 is rough / trial / a rangefinder / too far out / overshot ALLOW Titre 1 is an outlier / is anomalous Just very similar / within 0.05 / within 0.5 Titre 1 very different Just not accurate Titration 1 is a control experiment Acceptable Answers Reject Mark 28.00 (cm3) / 28.0 (cm3) / 28 (cm3) 28.14 (cm3) / 28.1 (cm3) / 28.13 (cm3) 6CH0201 1006<br />
ALLOW ANSWER IN EACH CASE WHETHER BY TE OR MARK SCHEME ANSWER, REGARDLESS OF IN (d)(i) to (d)(v), IGNORE UNITS EVEN IF INCORRECT AND ANY WORKING SHOWN Question Number 19 (d)(i) Question Number 19 (d)(ii) Question Number 19 (d)(iii) Question Number 19 (d)(iv) Acceptable Answers Reject Mark 0.100 x 28.00 = 0.0028 / 2.8 x 10 (mol) 1000 -3 ALLOW TE from (c) IGNORE sf except one sf Acceptable Answers Reject Mark 0.0028 / 2.8 x 10-3 (mol) OR Same answer to (d)(i) if TE applied IGNORE sf except one sf Acceptable Answers Reject Mark 0.0028 = 0.112 (mol dm-3) 0.025 OR Answer to (d)(ii) if TE applied from (d)(ii) 0.025 IGNORE sf except one sf Acceptable Answers Reject Mark 10 x 0.112 = 1.12 (mol dm-3) OR Answer to (d)(iii) x 10 if TE applied from (d)(iii) IGNORE sf except one sf 6CH0201 1006<br />
Question Number 19 (d)(v) Acceptable Answers 1.12 x 60 = 67.2 (g dm-3) OR Answer to (d)(iv) x 60 if TE applied from (d)(iv) IGNORE sf except one sf Reject Mark 67.1 6CH0201 1006<br />
Question Number 19 (e) Acceptable Answers Reject Mark NOTE: answer must refer to making up the diluted solution and not the titration NOTE: the Reason mark must be correctly linked to the Improvement Improvement: Use a pipette / burette to measure acid (solution) (1) Reason: Pipette / burette more accurate (than a measuring cylinder) (1) ALLOW more precise OR Improvement: Shake / invert the volumetric flask (thoroughly) (1) Reason: To ensure a uniform concentration (1) OR Improvement: Rinse out measuring cylinder (and transfer washings to the volumetric flask) (1) Reason: To ensure all the acid is transferred (to the volumetric flask) (1) OR Improvement: Use a (teat) pipette to make up to the mark (in volumetric flask) (1) Reason: To ensure volume of solution accurately measured (1) Use of volumetric flask for initial measurement of volume of vinegar solution more reliable swirl (the flask) to ensure fully dissolved just rinse out apparatus Any suggested improvements relating to the titration part of this experiment 6CH0201 1006<br />
Acceptable Answers Reject Mark Z / between 27.85 and 28.05 (cm3) ALLOW 27.95 0.10 (cm3) Acceptable Answers Reject Mark Any one of the following / a statement equivalent to:overshoots/misses end-pointwater left in burette / pipetteair lock below tap in burette / air in Question Number 19 (f)(i) Question Number 19 (f)(ii) pipette leaking tapburette not verticalalkali not at stated concentrationnot reading meniscus at eye-levelnot reading level against a white funnel left in top of burette backgroundnot reading meniscus correctlywashing pipette between titreswashing the flask with the solution that will go in itnot swirling flask / mixture IGNORE errors in calculation water left in conical flask just measurements may be inaccurate there could be uncertainty with other equipment contamination of the vinegar 6CH0201 1006<br />

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