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Question #44713

Difficulty: 8/10 | 🟢 Free

The Question

At $298\text{ K}$ , the vapor pressure of pure liquid A is $150\text{ Torr}$ and that of pure liquid B is $400\text{ Torr}$ . When a small amount of B is dissolved in A to form a dilute solution, the solvent A follows Raoult's Law, whereas the volatile solute B follows Henry's Law with a Henry's Law constant $K_H = 1900\text{ Torr}$ . If the vapor phase in equilibrium with the solution contains B at a mole fraction of $0.4$ , what is the total vapor pressure of the system?

Options

A.
237.5 Torr
B.
200.0 Torr
C.
162.5 Torr
D.
850.0 Torr

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

Henry’s Law Constant (Kh) Trends|Vapor Pressure Lowering (Mole Fraction)

Why This Question Is Tricky

Common Mistakes

This question tests the 'Lethal Collision' trap in solution thermodynamics where a system operates under a mixed regime (Raoult's Law for the solvent and Henry's Law for the volatile solute).

Students frequently fall into the trap of reflexively applying Raoult's Law to both components ( $P_{total} = P_A^\circ x_A + P_B^\circ x_B$ ) because the vapor pressure of pure B ( $P_B^\circ = 400\text{ Torr}$ ) is explicitly provided as a distractor.

Another common error is conflating the vapor phase mole fraction ( $y_B$ ) with the liquid phase mole fraction ( $x_B$ ), leading to an erroneous direct substitution.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Solutions - Class 12
Concept	Henry’s Law Constant (Kh) Trends
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free