Colligative Properties and Calculation of Molecular Mass

Chemistry Class XIIChapter Solutions

Colligative Properties and Calculation of Molecular Mass

"That property of a solution depend upon numbers of the present particles in the solution  is known as Colligative Properties."

As per Raoult's Law
"The relative lowering the vapour pressure is equal to the mole fraction of solute in the solution".

Let the vapour pressure of pure solvent and solvent in the solution are P⁰ and Ps respectively.

Lowering of vapour pressure =  P⁰ - Ps
Relative lowering of vapour pressure =  (P⁰ - Ps)/P⁰

Mole fraction of solute in the solution = n/(n + N)

Here,
n is the number of moles of the solute in the solution.
N is the number of moles of pure solvent.

According to Raulot's Law;

 (P⁰ - Ps)/P⁰ =  n/(n + N)
When n is far far less than N.

 (P⁰ - Ps)/P⁰ = n/N ...................... (i)
We know that
n = w/m = Weight of solute/ Molecular weight of solute
N = W/M = Weight of solvent/ Molecular weight of solvent

Now, from (i)

 (P⁰ - Ps)/P⁰ = wM/mW
Therefore, molecular weight of the solute

m = P⁰wM/W(P⁰ - Ps)

The above expression calculate the molecular mass of the solute.