State the general form of Raoult?s law and give a mathematical expression for the same. A
solution containing 15 g of non-volatile solute dissolved in 45 g of water has a vapour
pressure of 1400 Pa at 298 K. Further, 9 g of water is then added to the solution and the new
vapour pressure becomes 1450 Pa. Calculate the molar mass of the solu
Solution
Raoult's law states that for a solution of volatile liquids, the partial vapour pressure of each component in the solution is directly proportional to its mole fraction. Thus if there is a solution of two liquid components 1 and 2, then for component 1,
p1 ∝ x1
p1 = p1°x1
For component 2,
p2 = p2°x2
where
p1 , p2 = Partial vapour pressure of two volatile components 1 and 2 of the solution
p1° and p2° = Vapour pressure of pure components 1 and 2
x1 , x2 = Mole fractions of the components 1 and 2
According to Dalton's Law of partial pressure:
ptotal = p1 + p2
= p1°x1 + p2°x2
= (1-x2)p1° + p2°x2
ptotal = p1°+ ( p2°-p1°)x2
