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Question #100392

Difficulty: 8/10 | 🟢 Free

The Question

A buried iron pipe undergoes corrosion via a differential aeration cell in a moist soil environment at $298\text{ K}$ . The pipe surface develops a localized anodic pit of area $A_a = 10\text{ cm}^2$ surrounded by a large cathodic region of area $A_c = 1000\text{ cm}^2$ . The reduction of dissolved oxygen, $\ce{O2(g) + 2H2O(l) + 4e^- -> 4OH^-(aq)}$ , occurs uniformly over the cathodic region with a current density of $1.93\,\mu\text{A cm}^{-2}$ . Assuming the dissolution of iron, $\ce{Fe(s) -> Fe^{2+}(aq) + 2e^-}$ , is the sole anodic process and the system operates under steady-state conditions, what is the total mass of iron lost from the anodic pit over a period of $10^5\text{ s}$ ? (Given: Atomic weight of $\ce{Fe} = 56\text{ g mol}^{-1}$ )

Options

A.
$56\text{ mg}$
B.
$0.56\text{ mg}$
C.
$28\text{ mg}$
D.
$5.6\text{ g}$

Check Your Answer

Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Mechanism_Trap

Don't get tricked by the process! Learn the mechanical traps used in competitive exam questions.

Concepts Behind This Question

Equivalent Concept in Redox|Oxidation State Traps

Why This Question Is Tricky

Examiner Levers

Approximation Bait

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

Difficulty Boost : +3

Assumed Constant Trap

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

Difficulty Boost : +3

Common Mistakes

This question tests the Equivalent Concept in a localized corrosion scenario (Differential Aeration Cell).

The 'Hidden Variable' is the conservation of total current ( $I_c = I_a$ ) rather than current density ( $i_c \neq i_a$ ).

Students often fall into the trap of applying the cathodic current density directly to the anodic area (yielding $0.56\text{ mg}$ ) or misapplying the area ratio, failing to recognize that a large cathodic area drives a massive localized current density at the small anode, leading to rapid pitting corrosion.

The problem also omits Faraday's constant, requiring the student to recall $96500\text{ C mol}^{-1}$ , and uses mixed units (mg vs g) to test dimensional vigilance.

Another trap is using the $n$ -factor of oxygen ( $4$ ) instead of iron ( $2$ ), yielding $28\text{ mg}$ .

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Some Basic Concepts of Chemistry - Class 11 Part 1
Concept	Equivalent Concept in Redox
Difficulty	8
Cognitive Load	8
Target Solve Time	180 sec
Access	Free