What is the molar mass (in g/mol) of the unknown compound?

Question

Suppose that you add 26.0 g of an unknown molecular compound to 0.250 kg of benzene, which has a 

KfKf of 5.12 ^oC/m. With the added solute, you find that there is a freezing point depression of 2.75  ^oC compared to pure benzene.

Answer 1

The formula for freezing point depression is

ΔT = K_f M where M is the molality

For this case M = (26 / MM) / .25 where MM is the Molar mass of the unknown

Rearranging this and substituting for K_f and ΔT results in MM = 193.6 which rounds to 194 g/mole