1. The freezing point depression (∆T_f) is directly proportional to the molality of the solution.

Thus, ∆T_f = K_f · m

2. Suppose we prepare a solution by dissolving W₂ g of solute in W₁ g of solvent.

Moles of solute in W₁ g of solvent = √W₂/M₂,

where M₂ is the molar mass of the solute.

Mass of solvent = W₁ g = W₁/1000 kg

3. The molality is expressed as,

m = Moles of solute / Mass of solvent in kg,

m = (1000W₂ / (M₂ mol)) / (W₁/1000 kg)

4. Substituting equation (2) in equation (1), we get,

∆T_f = (1000K_fW₂) / (M₂W₁)

Hence,

M₂ = (1000K_fW₂) / (∆T_fW₁)