Question

The vapour pressure of water is 16.8 mm Hg at a certain temperature. If the vapour pressure of the solution is 16. 78 mm, find the molality of the solution.

Answer 1

P⁰ = 16.8 mm Hg; P = 16.78 mm Hg

Molar mass of water = M₁ = 18 g mol^-1

Molality of the solution = m = ?

`(P^0 - P)/P^0 = M_1/1000 xx m`

`∴ m = 1000/M_1 xx (P^0 - P)/P^0 = 1000/18 xx ((16.8 - 16.78)/16.87)`

 = 0.66 mol^-1kg

∴ Molality of the solution = 0.66 m.