Question

The relative lowering of vapour pressure of a dilute aqueous solution containing non-volatile solute is 0.0125. The molality of the solution is about ________.

• A. 0.70
• B. 0.50
• C. 0.90
• D. 0.80

Hint:

The relative lowering of vapour pressure of a dilute aqueous solution containing non-volatile solute is 0.0125. The molality of the solution is about ____.

Answer 1

The Correct Option is A

Step 1: Raoult's Law
Relative lowering of vapour pressure ($\frac{P-P_s}{P}$) is equal to the mole fraction of the solute ($\chi_{2}$).
Step 2: Values
$\chi_{2} = \frac{n}{n+N} = 0.0125$.
For a dilute solution, $n \ll N$, so $\frac{n}{N} \approx 0.0125$.
Step 3: Molality Formula
$\text{Molality} = \frac{n \times 1000}{W_{solvent}(\text{in g})} = \frac{n}{N \times M_{solvent}} \times 1000$.
Step 4: Calculation
Using $M_{water} = 18$: $\text{Molality} = \frac{0.0125}{18} \times 1000 = 0.0007 \times 1000 = 0.70 \text{ m}$.
Final Answer: (A)