From freezing point depression (most common): M = (Kf * w_solute * 1000)/(delta_Tf * w_solvent). Multiply by i if electrolyte is known. From boiling point elevation: M = (Kb * w_solute * 1000)/(delta_Tb * w_solvent). From osmotic pressure: M = w_solute * R * T / (pi * V * M_solute)... more directly: M_solute = (w_solute * R * T) / (pi * V) where V is in litres. Osmotic pressure method is preferred for macromolecules (polymers, proteins) because even dilute solutions give measurable osmotic pressure. Colligative method gives number-average molar mass for polydisperse systems. The "apparent" molar mass (ignoring i) differs from true molar mass — always calculate i first for electrolytes.