Question

10 g of a certain non-volatile solute was dissolved in 100 g of water at ${20}^{o}C$. The vapour pressure was lowered from 17.3555 to 17.235 mm. Calculate the molecular mass of the solute.

• A. 257.45
• B. 27.45
• C. 157.45
• D. 200.45

Answer 1

The correct option is A 257.45

We have lowering of vapour pressure

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

moles of water = 5.55moles

moles of non-volatile solute = x

so we have

$\frac{17.3555-17.235}{17.3555}=\frac{x}{x+5.55}$

x = 0.039

molecular weight of solute = $\frac{10}{0.039}$ = 257.45