Question

A solution is prepared by dissolving 10 g of a non-volatile solute in 360 g of water. What is the molar mass in $gmo{l}^{-1}$ of the solute if the relative lowering of vapour pressure of solution is $5\times {10}^{-3}$?

• A. 199
• B. 99.5
• C. 299
• D. 149.5

Answer 1

The correct option is B 99.5

$\frac{P^0-P}{P^0}=\frac{w\times M}{m\times W}$
where:
$w=$ weight of solute
$m=$ molar mass of solute
$W=$ weight of solvent
$M=$ molar mass of solvent
$5\times {10}^{-3}=\frac{10\times 18}{m\times 360}=100g$