Question

If a 0.4 molal solution of a nonvolatile solute in organic solvent decreases its freezing point by 2.4 K., then the molal depression constant of solvent will be

• A. \(3.0 \text{K kg mol}^{-1}\)
• B. \(4.0 \text{K kg mol}^{-1}\)
• C. \(5.0 \text{K kg mol}^{-1}\)
• D. \(6.0 \text{K kg mol}^{-1}\)

Hint:

Use delta Tf = Kf x m.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept
Depression of freezing point is a colligative property: \(\Delta T_f = K_f \, m\), where \(m\) is molality.

Step 2: Detailed Explanation
\[ K_f = \frac{\Delta T_f}{m} = \frac{2.4}{0.4} = 6.0 \text{ K kg mol}^{-1} \]
Options (A), (B), (C) would need a freezing point drop of 1.2 K, 1.6 K and 2.0 K respectively for a 0.4 molal solution, so they do not fit.

Final Answer:
\(K_f = 6.0 \text{ K kg mol}^{-1}\), option (D). \[ \boxed{6.0 \text{ K kg mol}^{-1}} \]