Question

The vapour pressure of water is 12.3 kPa at 300 K. Calculate vapour pressure of 1 molal solution of a non-volatile solute in it.

Answer 1

P°H₂O = 12.3 kPa

1000

In 1 molal solution,

n_solute = 1_; n_H2O = \(\frac{1000}{18}\) = 55.5

∴ ^χH₂O = \(\frac{55.5}{55.5+1}\)

Vapour pressure of the solution, 

P_s = P°H₂O ×  ^χH₂O

= 0.982 × 12.3 = 12.08 kPa.