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Question #44671
The Question
A 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 of \ce{O2} gas at STP ().
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 () 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 = , O = )
Options
- A.
\ce{Mn_3O_4},
- B.
\ce{Mn_3O_4},
- C.
\ce{Mn_3O_4},
- D.
\ce{Mn_2O_3},
Check Your Answer
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
| Subject | Chemistry |
| Chapter | Some Basic Concepts of Chemistry - Class 11 Part 1 |
| Concept | Empirical vs Molecular Formula |
| Difficulty | 8 |
| Cognitive Load | 7 |
| Target Solve Time | 160 sec |
| Access | Free |