Solutions

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,

po−Ps

po

= 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

n2

(n1+n2)

. Thus, we have

po−ps

po

= n

n2

n1+n2

Neglecting n2 (as n2 << n1) from the denominator, and substituting
po = 1 atm = 760 mm Hg, we get

760−ps

760

=

2.5∕MB

100∕18

×

1000

1000

=

18

1000

×m (1)
According to elevation in boiling point:
ΔTb = Kb×m
⇒ 2 = 0.76 × m ⇒ m =

2

0.76

Putting this value in Eq. (1), we get

760−ps

760

=

8

1000

×

2

0.76

⇒ ps = 760−

18×2×760

1000×0.76

= 760 - 36 = 724 mm Hg