Question

Explain two colligative properties.

Answer 1

1. Relative lowering of vapour pressure: When a non-volatile solute is added to a solvent, the vapour pressure of the solvent decreases. This is because some surface area is now occupied by solute particles, reducing the number of solvent molecules escaping into the vapour phase.
   `(p^circ - p)/p^circ = X_"solute"`
   

   Where:

   `p^circ` = vapour pressure of pure solvent

   p = vapour pressure of solution

   `X_"solute"`​ = mole fraction of solute

2. Elevation of boiling point: Addition of a non-volatile solute raises the boiling point of the solvent. This is because the solution has lower vapour pressure, so a higher temperature is needed for it to equal atmospheric pressure and boil.
   ΔT_b ​= K_b​ ⋅ m
   Where:
   ΔT_b​ = elevation in boiling point
   K_b = ebullioscopic constant
   m = molality of the solution