$w _{solvent}=75 g\quad\quad$ $M =$ Mol . Wt .of solute
$T _{ B }^{\prime}=373.535\, K$
$T _{ B }^{o}=373.15\, K$
$\Delta T _{ B }=0.385= K _{ b } \text { molality }$
$0.385=0.52 \times\left(\frac{2.5}{ M } \times \frac{1000}{75}\right)$
$M =45 \,g\, mol ^{-1}$
($k _{ f }=1.86\,K\,kg\,mol ^{-1}$ )