Question

$12$g of a non-volatile solute dissolved in $108$g of water produces the relative lowering of the vapour pressure of $0.1$. The molecular mass of the solute is:

• A. $80$
• B. $60$
• C. $20$
• D. $40$

Answer 1

Answer: (C)

$20$


$12$g of a non-volatile solute dissolved in $108$g of water produces the relative lowering of vapour pressure of $0.1$. The molecular mass of the solute is 20.
$\frac{\Delta P}{P^0}=0.1$
$\frac{\Delta P}{P^0}=\frac{W_2{M}_1}{W_1{M}_2}$
$0.1=\frac{\text{12 g}\times \text{18 g/mol}}{\text{108 g}\times M_2}$
$M_2=\text{20 g/mol}$
Note: $W_1$ and $W_2$ are weights of water and solute respectively.
$M_1$ and $M_2$ are molecular weights of water and solute respectively.