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Question #44718

Difficulty: 8/10 | 🟢 Free

The Question

At $340\text{ K}$ , $47.4\text{ g}$ of potash alum ( $\ce{KAl(SO4)2 \cdot 12H2O}$ ) is completely dissolved in $177.0\text{ g}$ of anhydrous chloral ( $\ce{CCl3CHO}$ ). Chloral reacts quantitatively with the released water of crystallization to form chloral hydrate, which acts as the sole liquid solvent. Assuming complete dissociation of the inorganic salt and ideal behavior of the resulting solution, what is the relative lowering of vapor pressure of the system? (Given molar masses in $\text{g/mol}$ : $\ce{K}=39$ , $\ce{Al}=27$ , $\ce{S}=32$ , $\ce{O}=16$ , $\ce{H}=1$ , $\ce{C}=12$ , $\ce{Cl}=35.5$ )

Options

A.
$\frac{1}{7}$
B.
$\frac{4}{7}$
C.
$\frac{1}{4}$
D.
$\frac{1}{13}$

Check Your Answer

Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Exception_Exposure

Learn to identify edge cases and boundary exceptions that JEE examiners hide in math problems.

Concepts Behind This Question

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

Why This Question Is Tricky

Common Mistakes

This problem tests the student's ability to recognize when water of crystallization acts as a chemical reactant rather than an inert co-solvent or solute.

The 'Hidden Variable' is the quantitative formation of chloral hydrate.

If a student treats the released water as an inert co-solvent, they will incorrectly calculate the total solvent moles as $1.2 + 1.2 = 2.4$ , leading to Option A (1/7).

If they treat water as a solute, they will get Option B (4/7).

If they forget the van't Hoff factor for the double salt, they will get Option D (1/13).

The correct approach requires understanding that the sequestration of water by chloral fundamentally alters the total number of moles in the liquid phase, combining two molecules into one.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Solutions - Class 12
Concept	Solubility of Solids in Liquids
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free