The properties which depend on the number of solute particles irrespective of their nature relative to the total number of particles in the solution are called colligative properties. These are relative lowering of vapour pressure of the solvent, depression of freesing point of the solvent, elevation of boiling point of the solvent and osmotic pressure of the solution. All of these properties help in calculating the observed molar mass of the solute.
Determination of osmotic pressure is considered best for the determination of molecular mass of the solute.
A solution of \(12.5\) g of an unknown solute in \(170\) g of water gave a boiling point elevation of 0.63 K .
\(k_b\) for water is \(0.53 K kg mol ^{-1}\).
The molar mass of the solute is :

1. A \(61.85 g mol ^{-1}\)
2. B \(87.3 g mol ^{-1}\)
3. C \(13.9 g mol ^{-1}\)
4. D \(56.7 g mol ^{-1}\)