Question

Calculate the molar mass of a nonvolatile solute if 6.4 g of it dissolved in 100 g water produces a relative lowering in vapour pressure of \(0\cdot 016\) at 300 K.

• A. \(60\text{ gmol}^{-1}\)
• B. \(66\text{ gmol}^{-1}\)
• C. \(72\text{ gmol}^{-1}\)
• D. \(84\text{ gmol}^{-1}\)

Hint:

Relative lowering equals the mole fraction of solute, which is about n2/n1 for a dilute solution.

Answer 1

The Correct Option is C

Step 1: Formula in terms of masses:
\[ \frac{\Delta p}{p^0} = \frac{w_2 M_1}{M_2 w_1} \]
Here $w_2 = 6.4$ g, $w_1 = 100$ g and $M_1 = 18$ g/mol.

Step 2: Rearrange:
\[ M_2 = \frac{w_2 M_1}{w_1\times 0.016} = \frac{6.4\times 18}{100\times 0.016} = \frac{115.2}{1.6} = 72 \]
So the answer is option (C).

Final Answer:
72 g per mole. \[ \boxed{72\text{ g mol}^{-1}} \]