It is clearly easy to convert between mole fraction and molality. Converting between Molarity and Molality. At first this seems to be an impossible task since we have already noted that we don't know the quantity of solvent we actually add to make a molar solution. However, it is relatively easy to measure the density of a solution. If we know the number of grams of solute dissolved in the 100 ml of solution, it is easy to subtract and determine the grams of solvent.
Density is thus the link between molarity and molality.