Molality:  Freezing Point Lowering

In dilute solutions, the lowering of the freezing point is directly proportional to the number of solute molecules (or moles) in a given weight of solvent.  The effect of a nonelectrolyte dissolved in water can be calculated by:

k_f x m  =  DT_f        (k_f  =  molal F.P. constant for H₂O  =  )

 (m  =  molality of the solution)

Then, the freezing point of the solution can be calculated by:  Freezing point of solvent -DT_f

   Example:  What is the freezing point of a solution containing 46.5 g of ethylene glycol  (MM = 62 g/mol) per 500 g (0.500 kg) of water?

k_f x m  =  x 1.50 m  =  2.79 ^oC   =  DT_f  

F.P. of water  - DT_f  =  0.000 ^oC   -  2.79 ^oC =  -2.79 ^oC