The technique used for determining the molecular mass of a substance by dissolving it and measuring the freezing point of the solution is called

The technique used for determining the molecular mass of a substance by dissolving it and measuring the freezing point of the solution is called Cryoscopy.
Cryoscopy is the measurement of the freezing point of a liquid using a cryoscope. The estimation of the molecular weight of a compound by measuring its effect on the freezing point of a solvent.
Helium is used in cryoscopy to obtain the very low temperatures required for superconductivity.