Colligative properties and determination of molar mass

Introduction :

The vapour pressure of solution decrease when a non volatile solution is added to a volation solvent. There are many properties of solution which are connected with this decreasing of vapour pressure. These are the relatively of vapour pressure of the solvent, depression of freezing points of the solvent. Elevation of the boiling point of the solvent, osmotic pressure of the solution. Everyone these of the property depend on the numeral of solute particle irrespective of their environment relative.

Relative lowering of vapour pressure:

In colligative properties and determination of molar mass, the vapour pressure of a solvent in solution is less than that of the pure solvent. Raoult recognized that the lower of vapour pressure depends simply on the concentration of the solute particles and it is dependent of their individuality.

`p_(1)=x_(1)p_(1)^(0)`

The reaction of the colligative properties in the vapour pressure of solvent is given as:

`Deltap_(1)=p_(1)^(0)-p_(1)=p_(1)^(0)-p_(1)^(0)x_(1)`

           =`p_(1)^(0)(1-x_(2))`

In a solution colligative properties containing several non volatile solutions, the lowering of the vapour pressure depends sum of the mole fraction of different solutions.

`(Deltap_(1))/(p_(1)^(0))` =`(p_(1^(0))-p_(1))/(p_(1)^(0))` =`x_(2)`

                The expression on the left hand side of the equation as mentioned earlier is called relative lowering of vapour force and is equal to the mole division of the solution of the colligative properties. The above equation can be determinations as:

`(p_(1)^(0)-p_(1))/(p_(1)^(0))` =`(n_(2))/(n_(1)+n_(2))`

                Here n₁ and n₂ are the number of mole of solvent and solute respectively present in the solution.

For dilute solution n₂<<n₁ hence neglection n₂ in the denominator we have

`(p_(1)^(0)-p_(1))/(p_(1)^(0))` =`(w_(2)xxM_(1))/(M_(2)xxW_(1))`

Here w₁ and w₂ are the mass and M₁ and M₂ are the molar mass of the solvent and solute correspondingly.

Elevation of boiling point:

In the colligative determination of molar mass, the vapour pressure of a liquid increase with enhance of temperature.It boils at the warmth at which its vapour pressure is identical to the atmospheric strain. The determination molar boiling point of a solution is always higher than that of the boiling point of the pure solvent.

Depression of freezing points:

The lowering vapour pressure of a solution cause a lowering of the freezing points compared to that of the determination  pure solvent in molar mass. The freezing points of the substance, the solids phase are the dynamic equilibrium with the liquid phases. The freezing points of the substance may be defined as temperature at which the vapour pressure of the substance in its liquid phase is equal to the vapour pressure in the solids state.