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Question #100935

Difficulty: 8/10 | 🟢 Free

The Question

Consider the following three redox processes occurring in an acidic aqueous medium at $298 \text{ K}$ :

Process 1: Spontaneous intramolecular redox (decomposition) of Chromium(VI) peroxide (\ce{CrO5}) to yield \ce{Cr^{3+}(aq)}, \ce{H2O(l)}, and \ce{O2(g)}. Process 2: Complete reduction of \ce{CrO5} by an excess of \ce{Fe^{2+}(aq)} ions, yielding \ce{Cr^{3+}(aq)}, \ce{Fe^{3+}(aq)}, and \ce{H2O(l)}. Process 3: Complete reduction of Caro's acid (\ce{H2SO5}) by an excess of \ce{I^{-}(aq)} ions, yielding \ce{SO4^{2-}(aq)}, \ce{I2(s)}, and \ce{H2O(l)}.

Let $n_1$ , $n_2$ , and $n_3$ be the n-factors (number of equivalents per mole) of the primary reactants (\ce{CrO5} in Processes 1 and 2; \ce{H2SO5} in Process 3). Which of the following correctly represents the values of $n_1$ , $n_2$ , and $n_3$ ?

Options

A.
$n_1 = 3.5$ , $n_2 = 7$ , $n_3 = 2$
B.
$n_1 = 7$ , $n_2 = 7$ , $n_3 = 2$
C.
$n_1 = 3$ , $n_2 = 3$ , $n_3 = 2$
D.
$n_1 = 3.5$ , $n_2 = 5$ , $n_3 = 4$

Check Your Answer

Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Numerical_Precision

Improve calculation stamina and avoid rounding errors in high-stakes competitive examinations.

Concepts Behind This Question

Isotopic Average Atomic Mass|Colligative Property Definition|Concentration Units Dependency

Why This Question Is Tricky

Examiner Levers

Approximation Bait

Options are extremely close; tests precision vs. early rounding.

Difficulty Boost : +3

Dimensional Analysis

Tests if the student verifies units/dimensions before selecting.

Difficulty Boost : +2

Assumed Constant Trap

Requires a constant not explicitly given (e.g., specific heat of water).

Difficulty Boost : +3

Common Mistakes

This question tests the 'Formal vs.

Real' oxidation state dichotomy and its impact on n-factor calculations in redox stoichiometry.

Students typically calculate oxidation states using the algebraic average method.

For species with peroxy linkages like \ce{CrO5} and \ce{H2SO5}, this yields mathematically correct but chemically fictitious states (Cr +10, S +8).

The trap is psychologically reinforced: the average method coincidentally gives the correct n-factor for \ce{H2SO5} ( $n_3 = 2$ ), lulling the student into a false sense of security.

However, applying it to the intramolecular decomposition of \ce{CrO5} fails catastrophically.

The true n-factor must be derived from the actual electron transfer counts based on the real oxidation states (Cr +6, O -1), revealing that $n_1 = 3.5$ , not 7.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Some Basic Concepts of Chemistry - Class 11 Part 1
Concept	Isotopic Average Atomic Mass
Difficulty	8
Cognitive Load	8
Target Solve Time	180 sec
Access	Free