Question

Which of the following statements are not correct?
A. For water, magnitude of \(K_b\) is more than the magnitude of \(K_f\).
B. The elevation in boiling point of water when a non-volatile solute is added to it is larger in magnitude than its depression in freezing point.
C. Osmotic pressure measurement is preferred over any other colligative property to determine molar mass of proteins and polymers.
D. The dimerised form of benzoic acid in benzene is C₆H₅-C-OH ⋯ O=C-C₆H₅

• A. A and B only
• B. A and D only
• C. A, B and D only
• D. A, C and D only

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Evaluate colligative properties statements. Knowledge of cryoscopic and ebullioscopic constants for water, the properties defining the measurement choices for macromolecules, and the specific molecular interaction geometry of dimerization is required.
Step 2: Key Formula or Approach:
1. Check standard values: For water, $K_f = 1.86\text{ K kg/mol}$, $K_b = 0.52\text{ K kg/mol}$.
2. Colligative magnitudes: $\Delta T_b = K_b m$ vs $\Delta T_f = K_f m$.
3. Dimer structural geometry of carboxylic acids.
Step 3: Detailed Explanation:
Let's analyze each statement:
A. For water, the molal freezing point depression constant is $K_f = 1.86\text{ K kg/mol}$ and the molal boiling point elevation constant is $K_b = 0.52\text{ K kg/mol}$. Thus, $K_f>K_b$. Statement A is incorrect.
B. For a given molality $m$ of a non-volatile solute, the elevation in boiling point is $\Delta T_b = K_b m$ and the depression in freezing point is $\Delta T_f = K_f m$. Because $K_f>K_b$, it follows that $\Delta T_f>\Delta T_b$. Statement B is incorrect.
C. Macromolecules (like proteins and polymers) have very high molar masses, so their molality is exceptionally small, making $\Delta T_b$ and $\Delta T_f$ difficult to measure accurately. However, osmotic pressure ($\pi$) relies on molarity and produces a large, easily readable pressure value even at dilute concentrations at room temperature. Thus, it is preferred. Statement C is correct.
D. Benzoic acid dimerizes in non-polar solvents like benzene by forming a stable cyclic 8-membered ring through TWO hydrogen bonds. The text structure shown in option D only depicts a single, incomplete linear hydrogen bond association instead of the classic double H-bonded ring structure. Therefore, Statement D is incorrect.
Since A, B, and D are incorrect, the correct option combining these is A, B, and D only.
Step 4: Final Answer:
Statements A, B, and D are incorrect.