Transition metal complex ions are coloured. The formula of a complex ion can be determined using colorimetry. 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. [Figure: Structure of C6H4(OH)CO2¯]

1. (a)(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 dm-³ solution. [A: Fe, 55.8; N, 14.0; Ο, 16.0; Η, 1.0] mass of Fe(NO3)3.9H2O = ................................. g[2]
2. (a)(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. In your answer you must give the name and capacity, in cm³, of any apparatus used.[2]
3. (b)(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 (x) to represent each data point. Draw two lines of best fit. [Figure: Graph grid][2]
4. (b)(ii)Use the graph in (i) to determine the volumes of Fe3+(aq) and aqueous 2-hydroxybenzoate which would give the maximum absorbance. volume of Fe3+(aq) = ................................. cm³ volume of aqueous 2-hydroxybenzoate = ................................. cm³[1]
5. (b)(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]
6. (b)(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]
7. (b)(v)Name the apparatus that should be used to measure the volumes of the solutions given in the table accurately.[1]
8. (c)In test 9, instead of mixing 8.0 cm³ of Fe3+(aq) and 2.0 cm³ of aqueous 2-hydroxybenzoate, the student mixed 16.0cm³ 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]
9. (d)In a colorimetry experiment, the absorbance of the solution follows the relationship shown. A = εcl A is the absorbance (no units). c is the concentration in mol dm-³. l is the path length of the light travelling through the solution in cm. ε is the molar absorption coefficient (a constant). Determine the unit of ε. unit = .................................[1]

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