OCR A Jun 2016 Paper 5 Q8

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8 Hydrated copper(II) methanoate, Cu(HCOO)2xH2O, is a copper salt. 18 A student carries out the procedure below to prepare Cu(HCOO)2xH2O and to determine the value of x in its formula. Step 1 The student prepares Cu(HCOO)2xH2O by reacting a copper compound with aqueous methanoic acid to form Cu(HCOO)2(aq) and allowing the solvent to evaporate. Step 2 The student dissolves 2.226 g of Cu(HCOO)2xH2O in water and makes up the solution to 250.0 cm3. Step 3 Using a pipette, the student adds 25.0 cm3 of this solution to a conical flask followed by an excess of KI(aq). The Cu2+(aq) ions react to form a precipitate of copper(I) iodide and I2(aq). In this reaction, 2 mol Cu2+ form 1 mol I2. Step 4 The student titrates the iodine in the resulting mixture with 0.0420 mol dm3 Na2S2O3 (aq). 2(aq) I2(aq) + 2S2O3 2I(aq) + S4O6 2(aq) 23.5 cm3 0.0420 mol dm3 Na2S2O3 (aq) is required to reach the end point. (a) Complete the electron configuration of copper in Cu(HCOO)2xH2O: copper(I) iodide: 1s21s2[2] (b) Choose a suitable copper compound for step 1, and write the full equation for the reaction that would take place to form Cu(HCOO)2(aq). State symbols are not required.[1] (c) Write an ionic equation, including state symbols, for the reaction in step 3.[1] (d) In step 4, the student adds a solution to observe the end point accurately. Name the solution and state the colour change at the end point. Solution added:Colour change:[2]OCR 2016<br /> (e) Determine the value of x in Cu(HCOO)2xH2O. Show your working. 19 [5] [Total: 11] END OF QUESTION PAPEROCR 2016<br />

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