Kb = R*$Tb^2$\frac{M_solvent}{1000*delta_H_vap}. Kf = R$Tf^2$*\frac{M_solvent}{1000*delta_H_fus}. These depend ONLY on the solvent. Higher Tb (or Tf) and lower delta_H_{vap} (or delta_H_{fus}) give larger constants. For water: Kb = 0.512 (small because delta_H_{vap} is large). Kf = 1.86 (larger because delta_H_{fus} is smaller). Camphor has Kf = 40 — extremely high because its delta_H_{fus} is low and Tf and $M_{solvent}$ are favorable. This makes camphor the best solvent for Rast's method of molar mass determination by freezing point depression. Higher K means larger measured $delta_T$ for the same molality, giving more precise results.