The freezing point is a colligative property. That implies that the way a solute affects it depends only in the number of particles in solution.
The number of particles from a solute is measured by the molality (the number of moles of solute per Kg of solvent) and the Van't Hoff factor. This last accounts for the number of ions (ion particles) per molecule of solute.
Then is the Van't Hoff factor what you need to determine for every solvent to predict which one will lower the freezing point of water the most.
