CIE Mar 2017 v2 Paper 5 Q2

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Transition metal complex ions are coloured. The formula of a complex ion can be determined using colorimetry. PMT In colorimetry, light of a certain wavelength is passed through a complex ion solution. The absorbance of the light is proportional to the intensity of the colour of the solution. The more concentrated the complex ion solution, the more intense its colour and so the higher the absorbance. A student carried out an experiment to determine the formula of the complex ion formed between aqueous iron(III) ions, Fe3+(aq), and aqueous 2-hydroxybenzoate ions, C6H4(OH)CO2 , which have the structure shown. O OH (a) In the first step of the experiment the student prepared 100.0 cm3 of 0.0500 mol dm3 aqueous iron(III) nitrate. (i) Determine the mass, in g, of solid hydrated iron(III) nitrate, Fe(NO3)3.9H2O, needed to prepare 100.0 cm3 of a 0.0500 mol dm3 solution. [Ar: Fe, 55.8; N, 14.0; O, 16.0; H, 1.0] (ii) Describe how, after weighing the mass determined in (i), the student should prepare 100.0 cm3 of 0.0500 mol dm3 aqueous iron(III) nitrate. mass of Fe(NO3)3.9H2O =g [2] In your answer you must give the name and capacity, in cm3, of any apparatus used.[2]UCLES 2017 9701/52/F/M/17 [Turn over<br /> 10 (b) The student prepared solutions containing various combinations of 0.0500 mol dm3 Fe3+(aq) and 0.0500 mol dm3 aqueous 2-hydroxybenzoate, as shown in the table. The student placed a small sample of each solution into a colorimeter and measured the absorbance. The student made a mistake in test number 9 and did not measure the result. PMT test number volume of Fe3+(aq) / cm3 1 0.0 volume of aqueous 2-hydroxybenzoate / cm3 10.0 0 absorbance 2 1.0 9.0 23 3 2.0 8.0 46 4 3.0 7.0 69 5 4.0 6.0 70 6 5.0 5.0 58 7 6.0 4.0 47 8 7.0 3.0 35 10 9.0 1.0 13 11 10.0 0.0 (i) Plot a graph on the grid to show the relationship between absorbance and the volumes of Fe3+(aq) and aqueous 2-hydroxybenzoate used. Use a cross () to represent each data point. Draw two lines of best fit. [2] absorbance 80 70 60 50 40 30 20 10 volume of Fe3+(aq) / cm3 volume of aqueous 2-hydroxybenzoate / cm3 0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0UCLES 2017 9701/52/F/M/17<br /> (ii) Use the graph in (i) to determine the volumes of Fe3+(aq) and aqueous 2-hydroxybenzoate which would give the maximum absorbance. 11 PMT volume of Fe3+(aq) =cm3 volume of aqueous 2-hydroxybenzoate =cm3 [1] (iii) The point of maximum absorbance shows where all of the ions are combined in the complex. Use the volumes in (ii) to determine the number of moles of 2-hydroxybenzoate ions that form a complex with 1 mole of Fe3+ ions. moles of 2-hydroxybenzoate ions =[1] (iv) Fe3+(aq) ions exist in aqueous solution as complex ions with the formula [Fe(H2O)6]3+(aq). 2-hydroxybenzoate ions, C6H4(OH)CO2 , are bidentate ligands. Use this information and your answer to (iii) to suggest the formula of the complex ion formed between Fe3+(aq) ions and 2-hydroxybenzoate ions.[1] (v) Name the apparatus that should be used to measure the volumes of the solutions given in the table accurately.[1]UCLES 2017 9701/52/F/M/17 [Turn over<br /> PMT 12 (c) In test 9, instead of mixing 8.0 cm3 of Fe3+(aq) and 2.0 cm3 of aqueous 2-hydroxybenzoate, the student mixed 16.0 cm3 of Fe3+(aq) and 4.0 cm3 of aqueous 2-hydroxybenzoate. Use your graph in (b)(i) to suggest the absorbance that would have been measured if a sample of this solution had been analysed in the colorimeter. absorbance =% [1] (d) In a colorimetry experiment, the absorbance of the solution follows the relationship shown. A = cl A is the absorbance (no units). c is the concentration in mol dm3. l is the path length of the light travelling through the solution in cm.is the molar absorption coefficient (a constant). Determine the unit of . unit =[1] [Total: 12] 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 2017 9701/52/F/M/17<br />

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