Answer
$500\frac{g}{mol}$
Work Step by Step
We know that
$m=\frac{\Delta T_f}{K_f}$
We plug in the known values to obtain:
$m=\frac{2.63}{40}=6.6\times 10^{-2}mol$
We also know that
$molar\space mass=\frac{mass}{no. \space of\space moles}$
We plug in the known values to obtain:
$molar \space mass=\frac{1.00g}{0.0250Kg\space solvent \times \frac{6.62\times 10^{-2}mol}{Kg\space solvent}}=500\frac{g}{mol}$
