The molar mass of the coumpound is 280 g/mol and its molecular formula is
So, you know that when you dissolve 100 mg of your unknown compound in 1 g of benzene, you get a lower freezing point than that of pure benzene.
This means that you can use the freezing-point depression formula to solve for the solution's molality.
For molecular compounds the van't Hoff factor is equal to 1.
Solve for the molality of the solution by
Determine the compound's molar mass by using its given mass
To determine its molecular formula, use its empirical formula
This will get you
Therefore, your compound's molecular formula is
SIDE NOTE If you go by the number of sig figs you gave for the mass of unknown compound dissolved in benzene, the answer should be 300 g/mol.
However, I'm quite sure that the answer's supposed to be 280, so I'll leave it with two sig figs.