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# At room temperature $25^{\large\circ}C$,the vapour pressure of pure methyl alcohol is 90 torr.Mol fraction of $CH_3OH$ in a solution in which vapour pressure of $CH_3OH$ is 20 torr at $25^{\large\circ}$ is ?

$\begin{array}{1 1}(a)\;0.30\\(b)\;0.22\\(c)\;0.40\\(d)\;0.50\end{array}$

$P^0=90$torr
$P=20$torr
$\large\frac{P^0-P}{P^0}=$$X=mole fraction of solute \large\frac{90-20}{90}=$$0.78$
$\therefore$ Mole fraction of solvent =1-0.78
$\Rightarrow 0.22$
Hence (b) is the correct answer.