OCR A Jan 2011 Paper 5 Q7

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20 The Dissolved Oxygen Concentration (DOC) in rivers and lakes is important for aquatic life. If the DOC falls below 5 mg dm3, most species of fish cannot survive. Environmental chemists can determine the DOC in water using the procedure below. A sample of river water is shaken with aqueous Mn2+ and aqueous alkali. The dissolved oxygen oxidises the Mn2+ to Mn3+, forming a pale brown precipitate of Mn(OH)3. O2(aq) + 4Mn2+(aq) + 8OH(aq) + 2H2O(l) 4Mn(OH)3(s) The Mn(OH)3 precipitate is then reacted with an excess of aqueous potassium iodide, which is oxidised to iodine, I2. 2Mn(OH)3(s) + 2I(aq) I2(aq) + 2Mn(OH)2(s) + 2OH(aq) The iodine formed is then determined by titration with aqueous sodium thiosulfate, Na2S2O3(aq). 2S2O3 2(aq) + I2(aq) S4O6 2(aq) + 2I(aq) A 25.0 cm3 sample of river water was analysed using the procedure above. The titration required 24.6 cm3 of 0.00100 mol dm3 Na2S2O3(aq). (a) (i) Calculate the DOC of the sample of river water, in mg dm3.OCR 2011 DOC = mg dm3 [4]<br /> (ii) Comment on whether there is enough dissolved oxygen in the river water for fish to survive. 21[1] (b) The presence of nitrate(III) ions, NO2 also oxidise iodide ions to iodine. , interferes with this method because NO2ions can During the reaction, a colourless gas is produced with a molar mass of 30 g mol1. (i) Predict the formula of the colourless gas.[1] (ii) Write an equation for the oxidation of aqueous iodide ions by aqueous nitrate(III) ions. Hydroxide ions are produced in this reaction.[2] [Total: 8] END OF QUESTION PAPEROCR 2011<br />

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