The depression in freezing point ($\Delta T_f$) is directly proportional to the molality ($m$) of the solute and the cryoscopic constant ($K_f$) of the solvent, given by the equation $\Delta T_f = K_f \cdot m$ . In molecular mass determination (specifically Rast's method), camphor is preferred as a solvent because it possesses a very high cryoscopic constant ($K_f \approx 39.7 \text{ K kg mol}^{-1}$). A high $K_f$ value ensures that even a small amount of solute produces a large, easily measurable depression in the freezing point, increasing the accuracy of the molar mass calculation.