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.
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 = `1000/18`
= 55.56 mol
Therefore, mole fraction of the solute in the solution is
`x_2= 1/(1+55.56)= 0.0177`
Vapour pressure of water, `p_1^0 =12.3 kPa`
Applying the relation, `(p_1^0-p_1)/p_1^0 = x_2`
`=> (12.3- p_1)/12.3 = 0.0177`
`=>12.3 - p_1 = 0.2177`
= 12.08 kPa (approximately)
Hence, the vapour pressure of the solution is 12.08 kPa.