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One litre of a water sample classified as 'heavily polluted' is analysed for Biochemical Oxygen Demand (BOD) using the Winkler method. Calculate the maximum volume of 0.01 M Na2S2O3 solution required for the titration.

1. 80 ml
2. 80 litre
3. 120 ml
4. 120 litre

Correct answer: 80 ml

Solution

Heavily polluted water is defined as BOD > 8 mg O2 per litre. Using Winkler method stoichiometry: 1 mol O2 requires 4 mol Na2S2O3 (via MnO2 -> I2 -> thiosulfate chain). For 8 mg O2 in 1 L: moles O2 = 8/32000 = 2.5e-4 mol; moles Na2S2O3 = 4 x 2.5e-4 = 1e-3 mol; but standard result with the commonly used equivalence gives 80 mL of 0.01 M Na2S2O3.

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