Two 25.0 mL portions of a solution containing Fe^2+ and Fe^3+ are taken. The first needs 12.0 mL of 0.0200 M KMnO4 in acid. The second is first shaken with zinc to reduce all iron to Fe^2+ (excess zinc removed) and then needs 30.0 mL of the same permanganate. Find the molarity of Fe^3+ in the original solution (three decimals).

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JEE Advanced levelNumerical answerDifficulty 3 of 3eq068

Two $25.0 mL$ portions of a solution containing $Fe X^{2 +}$ and $Fe X^{3 +}$ are taken. The first needs $12.0 mL$ of $0.0200 M$ $KMnO X_{4}$ in acid. The second is first shaken with zinc to reduce all iron to $Fe X^{2 +}$ (excess zinc removed) and then needs $30.0 mL$ of the same permanganate. Find the molarity of $Fe X^{3 +}$ in the original solution (three decimals).

Numerical answer type, answer in M, tolerance 0.001.

Answer: 0.072 M

First portion: Fe2+ = 5 x 0.0120 x 0.0200 = 1.20e-03 mol. Second portion: total iron = 5 x 0.0300 x 0.0200 = 3.00e-03 mol. Fe3+ = 3.00e-03 - 1.20e-03 = 1.80e-03 mol in 25.0 mL, so molarity = 1.80e-03/0.0250 = 0.072 M.

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Answer: 0.072 M

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