A solution made by dissolving 150.0 g of an unknown compound in 325.0 ml of benzene to make a solution which has a freezing point 18.6 °C lower than that of pure benzene. Kf for benzene is 5.12 °C/m and the density of benzene is 0.877 g/ml. What was the molality of the solution?
a) 1.24 m
b) 2.76 m
c) 3.89 m
d) 4.65 m