When a non-volatile solute is added to pure solvent, there is a decrease in vapour pressure. So, according to relative lowering in vapour pressure,
=
x2 where
x2 is the number of moles of solute,
po is the vapour pressure of the pure solvent and
ps is the vapour pressure of the solution. Now,
x2 can also represented as
. Thus, we have
=
n Neglecting
n2 (as
n2 << n1) from the denominator, and substituting
po = 1 atm = 760 mm Hg, we get
=
× =
×m (1)
According to elevation in boiling point:
ΔTb =
Kb×m ⇒ 2 = 0.76 × m ⇒ m =
Putting this value in Eq. (1), we get
=
× ⇒
ps =
760− = 760 - 36 = 724 mm Hg