OCR A Jun 2013 Paper 5 Q8

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8 Dimethylglyoxime, DMGH, can be used to analyse nickel(II) compounds. 18 An excess of a solution of DMGH is added to an acidic solution of a nickel(II) compound. Aqueous ammonia is added which precipitates out a nickel(II) complex, Ni(DMG)2, as a red solid. A sample of a hydrated nickel(II) salt is analysed using the procedure below. Step 1 2.50 g of the hydrated nickel(II) salt is dissolved in dilute acid. An excess of an aqueous solution of DMGH is added. Step 2 An excess of aqueous ammonia is added and the mixture is heated. A red precipitate of Ni(DMG)2 forms. An equation for the reaction is shown below. Ni2+(aq) + 2DMGH(aq) + 2NH3(aq) Ni(DMG)2(s) + 2NH4 red precipitate +(aq) Step 3 The red precipitate is filtered, washed with water, dried and then weighed. The precipitate of Ni(DMG)2 has a mass of 2.57 g. Assume that all Ni2+(aq) ions have been converted into Ni(DMG)2(s). M[Ni(DMG)2] = 288.7 g mol1. Step 4 A second 2.50 g sample of the hydrated nickel(II) salt is heated in a crucible to remove the water of crystallisation. 1.38 g of the anhydrous salt remains. (a) Complete the electron configurations of nickel as the element and in the +2 oxidation state. nickel as the element: 1s2 2s22p6nickel in the +2 oxidation state: 1s2 2s22p6[2]OCR 2013<br />
 19 (b) The structure of Ni(DMG)2 is shown below. H3C CH3 O H O Ni O H O H3C CH3 (i) State and explain the role of ammonia in step 2 of this experiment.[1] (ii) State the coordination number of Ni in Ni(DMG)2.[1] (iii) Why does the Ni(DMG)2 complex have no overall charge?[1] (iv) Draw the structure of dimethylglyoxime, DMGH. [1]OCR 2013 Turn over<br />
 (c) Determine a possible formula of the hydrated nickel(II) salt. 20 Your answer must show relevant working.OCR 2013 END OF QUESTION PAPER [7] [Total: 13]<br />

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