Calculate freezing point depression.
A dissolved solute cannot enter the growing crystal lattice, so it lowers the temperature at which the solid and liquid phases reach equilibrium. The size of the effect is set only by the concentration of dissolved particles, which is why road salt is effective: each formula unit of calcium chloride releases three ions and depresses the freezing point three times as much as an equal molality of a non-electrolyte.
Freezing point depression
delta Tf = i x Kf x molality; solution freezing point = normal freezing point - delta Tf
Because it releases three ions rather than two and is far more soluble, so it reaches a much lower practical working temperature — around -29 C compared with about -9 C for sodium chloride under road conditions.
There is a concentration beyond which adding more solute stops helping. For sodium chloride and water that eutectic is about 23 percent by mass at -21 C — the theoretical floor no amount of extra salt can beat.
Yes, that is cryoscopy. Measure the depression for a known mass of unknown solute in a known mass of solvent, solve for molality, and the molar mass follows. Assume i = 1 unless the solute is known to ionise.