Freezing point of solution is less than that of pure solvent, which of the following statements are correct ?
(A) Vapour pressure of solution is less than that of pure solvent
(B) Vapour pressure of solution is greater than that of pure solvent
(C) Only solvent molecules will freeze
(D) Only solute molecules will freeze

Question

$W$ g of a non-volatile electrolyte solid solute of molar mass $M\,\text{g mol}^{-1}$ when dissolved in $100\,\text{mL}$ water decreases vapour pressure of water from $640\,\text{mm Hg}$ to $600\,\text{mm Hg}$. If aqueous solution of the electrolyte boils at $375\,\text{K}$ and $K_b$ for water is $0.52\,\text{K kg mol}^{-1}$, then the mole fraction of the electrolyte $(x_2)$ in the solution can be expressed as ___.
(Given: density of water $=1\,\text{g mL}^{-1}$, boiling point of water $=373\,\text{K}$)

Answer 1

The question revolves around the properties of solutions, specifically related to the freezing point and vapor pressure. Let's analyze why the given statements and answer options are correct:

The freezing point of a solution is lower than that of the pure solvent because the presence of a solute affects the chemical potential of the solvent in the liquid phase. This is a colligative property known as freezing point depression.
Statement (A): "Vapor pressure of solution is less than that of pure solvent"
This is correct because when a non-volatile solute is added, it decreases the vapor pressure of the solvent. Therefore, the vapor pressure of the solution becomes lower than that of the pure solvent.
Statement (B): "Vapor pressure of solution is greater than that of pure solvent"
This statement is incorrect. As explained in Statement A, the vapor pressure decreases when a solute is added.
Statement (C): "Only solvent molecules will freeze"
This is correct because, during the freezing process, only the solvent crystallizes whereas the solute particles remain in the solution, hence lower freezing point.
Statement (D): "Only solute molecules will freeze"
This statement is incorrect because in freezing point depression caused by a solute, it is the solvent that undergoes solidification, not the solute molecules.

Based on the above explanations, the correct answer is A & C only, as these are the correct descriptions of the effect of solute on the freezing point and vapor pressure of a solution.

Answer 2

The question revolves around the properties of solutions, specifically related to the freezing point and vapor pressure. Let's analyze why the given statements and answer options are correct:

The freezing point of a solution is lower than that of the pure solvent because the presence of a solute affects the chemical potential of the solvent in the liquid phase. This is a colligative property known as freezing point depression.
Statement (A): "Vapor pressure of solution is less than that of pure solvent"
This is correct because when a non-volatile solute is added, it decreases the vapor pressure of the solvent. Therefore, the vapor pressure of the solution becomes lower than that of the pure solvent.
Statement (B): "Vapor pressure of solution is greater than that of pure solvent"
This statement is incorrect. As explained in Statement A, the vapor pressure decreases when a solute is added.
Statement (C): "Only solvent molecules will freeze"
This is correct because, during the freezing process, only the solvent crystallizes whereas the solute particles remain in the solution, hence lower freezing point.
Statement (D): "Only solute molecules will freeze"
This statement is incorrect because in freezing point depression caused by a solute, it is the solvent that undergoes solidification, not the solute molecules.

Based on the above explanations, the correct answer is A & C only, as these are the correct descriptions of the effect of solute on the freezing point and vapor pressure of a solution.

The Correct Option is B

Solution and Explanation

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