Question

Molecular weights of macromolecules are determined by :

• A. elevation of boiling point
• B. depression in freezing point
• C. osmotic pressure
• D. None of these

Answer 1

The correct option is C osmotic pressure

The number average molar mass can be determined by osmometry. The osmotic pressure of a solution is dependent only on the relative number of particles in the solution (in case of dilute solutions), irrespective of the nature (called colligative property).

Therefore, by taking certain known weight of the macromolecule, one wants to calculate the molecular mass of a solution is prepared and the osmotic pressure is measured. Then, this is compared to the known osmotic pressure of standard solution which gives the relative number of particles required to create such a pressure. From this and the weight of macromolecule used to prepare the solution, one can obtain the required molecular weight (weight of Avogadro's number of particles).