From freezing point depression (most common): M = $\frac{Kf * w_solute * 1000}{(delta_Tf * w_solvent)}$. Multiply by i if electrolyte is known. From boiling point elevation: M = $\frac{Kb * w_solute * 1000}{(delta_Tb * w_solvent)}$. 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.