1 molal solution means 1 mol of the solute is present in 1000 g of the solvent (water).

Molar mass of water = 18 g mol^-1

∴ Number of moles present in 1000 g of water = \(\frac{1000}{18}\)

= 55.56 mol

Therefore, mole fraction of the solute in the solution is x₂ = \(\frac{1}{1+55.56}\) = 0.0177

It is given that,

Vapour pressure of water, p₁⁰ = 12.3 kPa

Applying the relation, \(\frac{p_1^0 - p_1}{p_1^0} = x_2\)

⇒ \(\frac{12.3 - p_1}{12.3}\) = 0.0177

⇒ 12.3 - p₁ = 0.2177

⇒ p₁ = 12.0823

= 12.08 kPa (approximately)

Hence, the vapour pressure of the solution is 12.08 kPa.