Depression of Freezing Point

$\Delta T_f = T_f^\circ - T_f = K_f \cdot m$

where:

• $\Delta T_f$ = depression of freezing point (K or °C), always positive
• $T_f^\circ$ = freezing point of pure solvent; $T_f$ = freezing point of solution ($< T_f^\circ$)
• $K_f$ = cryoscopic constant (molal freezing point depression constant) — depends only on the solvent
• $m$ = molality of solution

Expanded form:
$\Delta T_f = K_f \times \frac{w_2 \times 1000}{M_2 \times w_1}$

Solving for molar mass:
$M_2 = \frac{K_f \times w_2 \times 1000}{\Delta T_f \times w_1}$