If the solution is treated as an ideal solution, the extent of freezing 
point depression depends only on the solute concentration that can be 
estimated by a simple linear relationship with the cryoscopic constant:
ΔTF = KF · m · i
ΔTF, the freezing point depression, is defined as TF (pure solvent) - TF
 (solution).
KF, the cryoscopic constant, which is dependent on the properties of the
 solvent, not the solute. Note: When conducting experiments, a higher KF
 value makes it easier to observe larger drops in the freezing point. 
For water, KF = 1.853 K·kg/mol.[1]
m is the molality (mol solute per kg of solvent)
i is the van 't Hoff factor (number of solute particles per mol, e.g. i =
 2 for NaCl).