Question

$12g$ of a non-volatile solute dissolved in $108g$ of water produces the relative lowering of vapour pressure of $\mathrm{0.1.}$ 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 $0.1=\frac{12}{m}\times \frac{18}{108}$
$m=\frac{12\times 18}{10.8}=20$