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Question #44671

Difficulty: 8/10 | 🟢 Free

The Question

A 2.29 g2.29 \text{ g} sample of a pure oxide of manganese, \ce{Mn_xO_y}, is fused with excess \ce{KOH} in the presence of \ce{O2} gas. The oxide is completely converted to a green melt, consuming exactly 560 mL560 \text{ mL} of \ce{O2} gas at STP (22.4 L/mol22.4 \text{ L/mol}).

The green melt is extracted with water and acidified with dilute \ce{H2SO4}, causing complete disproportionation of the green species into a black precipitate and a purple solution.

After filtering the black precipitate, the purple filtrate is adjusted to a faintly alkaline pH (pH≈8pH \approx 8) and treated with excess \ce{KI}. This results in the complete reduction of the purple species, forming more of the black precipitate and a soluble iodine-containing species.

Identify the empirical formula of the original oxide \ce{Mn_xO_y} and calculate the exact number of moles of the soluble iodine-containing species formed.

(Assume atomic masses: Mn = 55 u55 \text{ u}, O = 16 u16 \text{ u})

Options

  • A.

    \ce{Mn_3O_4}, 0.01 mol0.01 \text{ mol}

  • B.

    \ce{Mn_3O_4}, 0.05 mol0.05 \text{ mol}

  • C.

    \ce{Mn_3O_4}, 0.03 mol0.03 \text{ mol}

  • D.

    \ce{Mn_2O_3}, 0.01 mol0.01 \text{ mol}

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Mechanism_Trap

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Concepts Behind This Question

Empirical vs Molecular Formula|KMnO₄ Redox pH Dependency|Disproportionation Constraints

Why This Question Is Tricky

Common Mistakes

This problem tests two major hidden variables in transition metal redox chemistry.

First, the pH-dependent disproportionation of the manganate ion (\ce{MnO4^{2-}}) in acidic media, which dictates the stoichiometric yield of permanganate.

Second, the exceptional pH-dependent oxidation of iodide by permanganate; while students habitually default to forming \ce{I2} (or \ce{I3^-}) as in acidic titrations, faintly alkaline or neutral conditions uniquely oxidize iodide all the way to iodate (\ce{IO3^-}).

Failing to account for this specific alkaline pathway leads to incorrect molar ratios.

Quick Facts About This Question

Question Information

SubjectChemistry
ChapterSome Basic Concepts of Chemistry - Class 11 Part 1
ConceptEmpirical vs Molecular Formula
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree