Question

$12\, g$ 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

• A. 80
• B. 60
• C. 20
• D. 40

Answer 1

Answer: (C)

From Raoults law, relative lowering in vapour pressure
$ \Delta p=\frac{{{p}^{o}}-p}{{{p}^{o}}}=\frac{n}{N}=\frac{W}{m}\times \frac{M}{W} $
Here, $ w=12g;W=108g,.m=? $
$ M=18g,\Delta p=0.1 $
$ \Delta p=\frac{W}{m}\times \frac{M}{W} $
or $ 0.1=\frac{12}{m}\times \frac{18}{108} $
$ m=\frac{12\times 18}{10.8}=20 $