Question

When 25g of a non-volatile solute is dissolved in 100g of water, the vapour pressure is lowered by 0.225 mm. If the vapour pressure of water at ${25}^{o}C$ is 17.5 mm, what is the molecular mass of the solute?

• A. 200
• B. 300
• C. 350
• D. 250

Answer 1

The correct option is C 350

We have,

$\frac{P_B^0-P_t}{P_B^0}=x_A$ = $\frac{n_A}{n_A+n_B}$

on putting the values we have,

$\frac{0.225}{17.5}=\frac{x}{x+5.55}$

we get x = 0.072

so, molecular weight = $\frac{givenweight}{no.ofmoles}$

so, molecular weight = 350