where $w_{1}, \,M_{1}=$ mass in $g$

and mol. mass of solvent

$w_{2}, M_{2}=$ mass in $g$

and mol. mass of solute

Let $M_{2}=x$

$P^{o}=185\,torr$ ;  $P_{s}=183\,torr$

$\frac{185-183}{183}=\frac{1.2 \times 58}{100 x}$

(Mol. mass of acetone $=58$ )

$x=64$

Molar mass of substance $=64$