Question

The vapour pressure of water is 17.54 mm hg at 293 k. calculate vapour pressure of 0.5 molal solution of a solute in it.

Answer 1

0.5 molal solution contains 0.5 mol of solute in 1000 ml of water
vapour pressure of pure water , P⁰ = 17.54 mm hg 
no. of moles of water = 1000g/18 = 55.55 mol
no.  of moles of solute = 0.5 mol

hence mole fraction of water = 55.55/(55.55+0.5) = 55.55/56.05 = 0.99
mole fraction of solute = 1- 0.99 = 0.01 

vapour pressure of water , P = P°Xa = 17.54 X 0.99 =  17.36 mm hg 

hence , vapour pressure of water in the solution is 17.36 mm hg