OCR A Jun 2015 Paper 5 Q7

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7 Chromite is the main ore of chromium. The chromium-containing compound in chromite is Fe(CrO2)2. The percentage of chromium in a sample of chromite can be determined using the method below. 22 Step 1 A 5.25 g sample of chromite ore is heated with sodium peroxide, Na2O2. 2Fe(CrO2)2 + 7Na2O2 2NaFeO2 + 4Na2CrO4 + 2Na2O Water is added to the resulting mixture. Na2CrO4 dissolves in the water forming a solution containing CrO4 2 ions. Step 2 Step 3 The mixture from Step 1 is filtered and the filtrate is made up to 1.00 dm3 in a volumetric flask. A 25.0 cm3 sample of this alkaline solution is pipetted into a conical flask and an excess of aqueous potassium iodide is added.A redox reaction takes place between I ions, CrO4 In this reaction 1 mol CrO4 2 forms 1.5 mol I2. 2 ions and H2 O. The resulting mixture is titrated with 0.100 mol dm3 sodium thiosulfate, Na2S2O3(aq) to estimate the I2 present: I2(aq) + 2S2O3 2(aq) 2I(aq) + S4O6 2(aq) The average titre of Na2S2O3(aq) is 25.5 cm3. (a) In Step 1 Na2O and NaFeO2 react with water forming an alkaline solution containing a brown precipitate. This is not a redox reaction. Write equations for: the reaction of Na2O with water the reaction of NaFeO2 with water.[2]OCR 2015<br />
 (b) Determine the percentage, by mass, of chromium in the ore. Give your answer to one decimal place. 23 (c) This part refers to Step 2 of the method. [6] 2 ions and H2 O: In the redox reaction between I ions, CrO4CrO42 ions, are reduced to chromium(III) ions, Cr3+ I ions are oxidised to iodine, I2 Construct an overall equation for the redox reaction and write half equations for the oxidation and reduction. Overall equation: Half equations:OCR 2015 END OF QUESTION PAPER [3] [Total: 11]<br />

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