Question

The relative lowering of vapour pressure caused by dissolving $71.3g$ of a substance in $1000g$ of water is $7.13\times {10}^{-3}$. The molecular mass of the substance is :

• A. $178.72$
• B. $18$
• C. $1.8$
• D. $360$

Answer 1

Answer: (A)

$178.72$

The relative vapour pressure caused by dissolving a substance in water is given as,

$\frac{P^0-P_s}{P^0}=\left(\frac{\frac{w}{m}}{\frac{w}{m}+\frac{W}{M}}\right)$

$7.13\times {10}^{-3}=\left(\frac{\frac{71.3}{m}}{\frac{71.3}{m}+\frac{1000}{18}}\right)$

$m=178.72$

Thus, the molecular mass of the substance is $178.72$.

Hence, the correct option is $A$