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Which of the following statements about redox reactions/titrations are INCORRECT? (A) 0.2 M, 500 mL KMnO4 solution will oxidise 0.08 mol of I- in alkaline medium. (B) Iodometric titration is used for estimation of a reducing agent in a sample. (C) Combustion of CH4(g) is an example of a redox reaction. (D) In the reaction a Cu + b HNO3 -> Cu(NO3)2 + NO + H2O, the ratio b/a = 8/3.

1. 0.2 M, 500 mL KMnO4 solution will oxidise 0.08 mol of I- in alkaline medium.
2. Iodometric titration is used for estimation of a reducing agent in a sample.
3. Combustion of CH4(g) is an example of a redox reaction.
4. a Cu + b HNO3 -> Cu(NO3)2 + NO + H2O, then b/a is 8/3.

Correct answer: 0.2 M, 500 mL KMnO4 solution will oxidise 0.08 mol of I- in alkaline medium.

Solution

In alkaline medium KMnO4 acts as a 1-electron oxidant (Mn7+ -> Mn6+). Moles = 0.1; equivalents = 0.1. I- -> (1/2)I2, losing 1e- per I-. So 0.1 mol KMnO4 oxidises 0.1 mol I-, not 0.08 mol. Statement A is incorrect. Statement B is correct (iodometry estimates reducing agents). Statement C is correct (CH4 + 2O2 -> CO2 + 2H2O: C goes from -4 to +4). Statement D: 3Cu + 8HNO3(dilute) -> 3Cu(NO3)2 + 2NO + 4H2O; b/a = 8/3. Correct.

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