Question

Find the molality of a solution containing a non-volatile solute if the vapour pressure is 2% below the vapour pressure of pure water.

Answer 1

Let P° be the vapour pressure of water.

Ps = vapour of the solution.

Formulae :

( P° - Ps) / Ps = (W × M) / (m ×W)


w / (m × W) × 1000 × M / 1000

Molality = (W ×1000) / (m × W)


(P° - Ps) / Ps = Molality × M/1000

Ps = 0.98P°

Substituting in the formulae :

(P° - 0.98P°) / 0.98P° = molality × 1000 / 18

Molality = 0.02 P° / 0.98 P° × 1000 /18

Molality = 1.133