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

Given: 1 molal solution means 1 mole of solute present in 1000g of water solvent)

Molecular weight of water = H₂O = 1 × 2 + 16 = 18g/mol

No. of moles of water, n = given mass /molecular weight

⇒ n = 1000/18 = 55.56 gmol^-1

Mole fraction of solute in solution,x₂ = moles of solute/(moles of solute + moles of water)

⇒ x₂ = 1/(1 + 55.56)

⇒ x₂ = 0.0177

Given vapour pressure of pure water at 300k is 12.3 kpa

Apply the formula:

⇒

⇒ P₁ = 12.0823kpa

which is the vapour pressure of the solution.