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Question 54

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 

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Choose the correct answer from the options given below :

Solution

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Statement A: (Incorrect)

  • For water, the molal boiling point elevation constant ($$K_b$$) is approximately $$0.52 \text{ K kg mol}^{-1}$$.
  • The molal freezing point depression constant ($$K_f$$) is approximately $$1.86 \text{ K kg mol}^{-1}$$.
  • Therefore, the magnitude of $$K_b$$ ($$0.52$$) is less than the magnitude of $K_f$ ($$1.86$$).

    Statement B: (Incorrect)
  • The relationships for colligative properties are given by:

    $$\Delta T_b = K_b \cdot m$$

    $$\Delta T_f = K_f \cdot m$$

  • Since $$K_b < K_f$$ for water, for any given molality ($$m$$) of a non-volatile solute, the elevation in boiling point ($$\Delta T_b$$) will always be smaller in magnitude than the depression in freezing point ($$\Delta T_f$$).

    • Statement C: (Correct)

  • Biomolecules like proteins and polymers have very high molar masses, meaning their solutions are extremely dilute in terms of molarity/molality. As a result, changes like $$\Delta T_b$$ or $$\Delta T_f$$ are too small to be measured precisely.
  • Osmotic pressure ($$\Pi = CRT$$) yields significantly larger, easily measurable values even at highly dilute concentrations and can be safely measured around room temperature ($$298\text{ K}$$) without denaturing the proteins.

    • Statement D: (Correct)

    In a non-polar solvent like benzene, benzoic acid undergoes dimerization due to intermolecular hydrogen bonding to form a stable cyclic structure:

    $$(C_6H_5COOH)_2$$

    statements A and B are the incorrect ones.

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