CIE Jun 2015 v2 Paper 5 Q2

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In order to identify a monoprotic (monobasic) hydroxycarboxylic acid, HX, the following experiments are carried out. 25.0 cm3 of an aqueous solution of HX is titrated against 0.0500 mol dm3 aqueous sodium carbonate. The end-point of the titration is reached when 25.0 cm3 of the aqueous sodium carbonate has been added. (a) (i) Write the equation for the complete neutralisation of HX with sodium carbonate.[1] (ii) How does the equation show that the concentration of HX is 0.100 mol dm3?[1] (b) (i) State the acid dissociation constant, Ka, for the above reaction in terms of H+ and HX only. Ka = (ii) The pH of the aqueous solution of HX is 2.43. Use the pH and the concentration of HX to show that the pKa of the acid is 3.86. All your working must be shown. [1]UCLES 2015 9701/52/M/J/15 [2] [Turn over<br /> (c) In an experiment various masses of the sodium salt of the acid, NaX, are added to separate portions of 100 cm3 of HX with stirring. After each addition the pH of the solution obtained is measured. The results of the experiment are recorded in the table below. mass of NaX added / g 0.00 0.10 0.20 0.30 0.40 0.60 0.80 1.00 1.20 1.50 2.00 pH 2.43 2.81 3.11 3.19 3.41 3.59 3.71 3.81 3.89 3.99 4.11UCLES 2015 9701/52/M/J/15<br /> (i) Plot a graph to show how the pH of the solution varies with the mass of NaX added. Draw the curve of bestt. 4.2 4.0 3.8 3.6 3.4 pH 3.2 3.0 2.8 2.6 2.4 0.2 0.4 0.6 1.0 0.8 1.2 mass of NaX added / g 1.4 1.6 1.8 2.0 [2] (ii) Circle one anomalous point on your graph and give a reason for how this could have occurred using the experimental procedure described.[2]UCLES 2015 9701/52/M/J/15 [Turn over<br /> (d) (i) The graph shows that a pH of 3.86 is obtained when 1.12 g of NaX is added to 100 cm3 of HX. Remember that pKa of HX is also 3.86. Use this information to calculate the relative molecular mass, Mr, of HX. Show your working. [Ar: H, 1.0; C, 12.0; O, 16.0; Na, 23.0] (ii) The calculated Mr is subject to small experimental error. Suggest a structure for the organic hydroxycarboxylic acid, HX, that bestts your Mr data. If you have not calculated a value for the Mr, use the value of 104. This is not the correct value. [3] [1] (e) Another method for determining the concentration of the acid HX could be to evaporate a sample of the solution and weigh the solid that remains. Suggest two reasons why this might not be a very good method ofnding the mass of solid HX in a sample of the solution.[2] [Total: 15] Permission to reproduce items where third-party owned material protected by copyright is included has been sought and cleared where possible. Every reasonable effort has been made by the publisher (UCLES) to trace copyright holders, but if any items requiring clearance have unwittingly been included, the publisher will be pleased to make amends at the earliest possible opportunity. To avoid the issue of disclosure of answer-related information to candidates, all copyright acknowledgements are reproduced online in the Cambridge International Examinations Copyright Acknowledgements Booklet. This is produced for each series of examinations and is freely available to download at www.cie.org.uk after the live examination series. Cambridge International Examinations is part of the Cambridge Assessment Group. Cambridge Assessment is the brand name of University of Cambridge Local Examinations Syndicate (UCLES), which is itself a department of the University of Cambridge.UCLES 2015 9701/52/M/J/15<br />

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