$delta_{Tf}$ = i * Kf * m. Kf = molal cryoscopic constant = R*$Tf^2$*\frac{M_solvent}{1000*delta_H_fus}. For water: Kf = 1.86 K.kg/mol, Tf = 273 K. Physical explanation: solute particles interfere with crystal lattice formation, requiring a lower temperature for freezing. $delta_{Tf}$ is always positive (freezing point drops). Applications: antifreeze (ethylene glycol in car radiators), salting icy roads (NaCl lowers freezing point). Kf > Kb for water because delta_H_{fus} < delta_H_{vap} (less energy involved in fusion, so the effect per particle is larger). Camphor is the best solvent for cryoscopy (Kf = 40 K.kg/mol, very high sensitivity). Molar mass formula: M = (i * Kf * $w_{solute}$ * 1000) / ($delta_{Tf}$ * $w_{solvent}$).