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Question #44545

Difficulty: 8/10 | 🟢 Free

The Question

An amount $W$ of Mohr's salt, $\ce{(NH4)2Fe(SO4)2 \cdot 6H2O}$ , is dissolved in $94.6\text{ g}$ of water at $298\text{ K}$ . The resulting solution exhibits a boiling point elevation of $1.30\text{ K}$ . Assuming complete dissociation of the double salt into its constituent ions, what is the value of $W$ ?

(Given: $K_b$ for water $= 0.52\text{ K kg mol}^{-1}$ , Molar mass of Mohr's salt $= 392\text{ g mol}^{-1}$ )

Options

A.
19.6 g
B.
18.5 g
C.
14.2 g
D.
34.0 g

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

Van’t Hoff Factor (i) Application|Hydrate Stability vs. Carbonyl|Solubility of Solids in Liquids

Why This Question Is Tricky

Common Mistakes

This problem tests two critical traps in colligative properties.

First, the 'Hidden Variable' of water of crystallization: students frequently forget that the water molecules in a hydrate become part of the solvent upon dissolution, which alters the total solvent mass and decreases the actual molality.

Second, the distinction between a double salt and a coordination complex: Mohr's salt is a double salt that completely dissociates into 5 ions ( $i=5$ ), whereas students might incorrectly treat it as a complex ion with a lower Van't Hoff factor (e.g., $i=3$ ).

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Solutions - Class 12
Concept	Van’t Hoff Factor (i) Application
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free