Question

Which of following is correct?

• A. the lowering of vapour pressure is equal to the mole fraction of solute
• B. the relative lowering of vapour pressure is equal to the mole fraction of solute
• C. the relative lowering of vapour pressure is proportional to the amount of solute in solution
• D. the vapour pressure of the solution is equal to the mole fraction of solvent

Hint:

Do not confuse "lowering of vapour pressure" (\( P^0 - P_s \)) with "relative lowering of vapour pressure" (\( \frac{P^0 - P_s}{P^0} \)). Only the relative lowering is equal to the mole fraction of the solute.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the correct statement regarding Raoult's law for solutions containing non-volatile solutes.

Step 2: Detailed Explanation:
According to Raoult's Law, when a non-volatile solute is added to a solvent, the vapour pressure of the solvent decreases.
The lowering of vapour pressure is given by:
\[ \Delta P = P^0 - P_s \] where \( P^0 \) is the vapour pressure of the pure solvent and \( P_s \) is the vapour pressure of the solution.
The relative lowering of vapour pressure is defined as:
\[ \frac{P^0 - P_s}{P^0} \] According to Raoult's law, this relative lowering of vapour pressure is equal to the mole fraction of the solute (\( \chi_{\text{solute}} \)):
\[ \frac{P^0 - P_s}{P^0} = \chi_{\text{solute}} \] Hence, option (B) is the correct statement.

Step 3: Final Answer:
(B) the relative lowering of vapour pressure is equal to the mole fraction of solute