At the same cost per pound, which alcohol, CH3OH or C2H5OH, would be more economical to purchase as an antifreeze for your car? Why?
delta T = Kf*molality
molality = moles/kg solvent
moles = grams/molar mass.
Go through the exercise. The higher molality is cheaper, molality depends on moles and moles depends upon grams and molar mass. For the same mass, the lower molar mass gives higher moles. Which has the lower molar mass?