## Raoult’s law source : chem wiki.uc davis.edu

## Raoult’s law – Numerical

### Question1) The vapour pressure of water at 200C is 17.54 mm. When 20 gm. of a nonvolatile solute is dissolved in 100 gm. of water, the vapour pressure is lowered by 0.30 mm what is the molecular weight of substance?

Solution:

p=17.54 mm.

p-ps =0.30

w = 20 gm

M of H2O = 2+16=18

m = ?

W= 100gm.

(p-ps) /p =wM/Wm

0.30/17.54 =20 x18 /100m

## m =210.48 Ans.

### Question2 ) The vapour pressure of pure Benzene at  certain temp. is 640 mm Hg. A non volatile non electrolyte solid weighing 2.175 gm. is added to 39.0 gm. of Benzene. The vapour pressure of solution is 600 mm Hg. What is the molecular weight of solid substance?

Solution:

p = 640 mm

w= 2.175 gm

W= 39.0 gm

ps=600 mm

m=?

M of benzene = 12×6+1×6 = 72+6 = 78

(p-ps) /p =wM/Wm

( 640-600)/640 = (2.175 x 78 ) / 39 m

## m= 69.6 Ans.

### Solution:

p =760 mm

ps =750 mm

( p-ps) / p = mole fraction of solute (X1)

760-750/750 =X1

X1 = 10/750