13012 10 Which one of the following statements is the definition of the base peak in a mass spectrum? A The highest mass ion B The peak of greatest abundance C The lowest m/z peak D The molecular ion peak Answer                 [1] 13012 BLANK PAGE DO NOT WRITE ON THIS PAGE (Questions continue overleaf) [Turn over 13012 Section B Answer all five questions in this section 11 The ethanedioate ion, C2O2− 4  , may act as a bidentate ligand. Ethanedioate ions, C2O2− 4  , can be titrated using manganate(VII) ions in solution. (a) Explain the term bidentate ligand. [2] (b) The reaction below occurs when a solution containing ethanedioate ions is added to a solution containing hexaaquanickel(II) ions. [Ni(H2O)6]2+  +  3C2O2− 4  →  [Ni(C2O4)3]4-  +  6H2O (i) Suggest why the enthalpy change for this reaction is approximately zero. [2] (ii) Explain, with reference to the equation, why this reaction occurs despite the enthalpy change being approximately zero. [2] 13012 [Turn over (iii) State the co-ordination number and oxidation state of nickel and the shape of the [Ni(C2O4)3]4- complex. co-ordination number: oxidation state of nickel: shape of complex: [3] (c) Hexaaquachromium(III) ions react with ethanedioate ions in solution. The complex formed is shown below. This complex exhibits isomerism. The cis (Z) isomer is shown below and is blue-grey in solution. The trans (E) isomer is purple in solution. Cr O O OH2 O OH2 O C O C O C C O O (i) Write an equation for the formation of this complex from hexaaquachromium(III) ions. [2] (ii) Suggest the structure of the trans (E) isomer. [1] 13012 (d) A 25.0 cm3 portion of a solution of iron(II) ethanedioate was acidified using an excess of sulfuric acid and titrated with 0.0150 mol dm-3 potassium manganate(VII) solution. The overall equation for the reaction is: 10FeC2O4 + 6KMnO4 + 24H2SO4 →  6MnSO4 + 5Fe2(SO4)3 + 3K2SO4 + 24H2O + 20CO2 (i) Explain in terms of oxidation states why this is a redox reaction. [4] (ii) Write three half-equations to show the oxidation and reduction reactions occurring in this overall reaction. [3] 13012 [Turn over (iii) State the colour change which would be observed at the end point during this titration. [1] (iv) The mean titre was found to be 12.45 cm3. Calculate the concentration of the iron(II) ethanedioate solution in mol dm-3. Give your answer to an appropriate number of significant figures. [4] (v) The iron(II) ethanedioate solution was originally made by dissolving 0.561 g of hydrated iron(II) ethanedioate (FeC2O4.xH2O) in 250 cm3 of deionised water. Calculate the value of x in FeC2O4.xH2O. [3]