Question

State Raoult’s law for the solutions of non-volatile solutes in volatile solvents.

State and explain Raoult’s law for solutions containing non-volatile solutes.

Answer 1

The vapour pressure of a solution containing a non-volatile solute at a particular temperature is directly proportional to the mole fraction of the solvent in the solution.

According to Raoult’s law,

`P = P_"solvent" = P^circ * chi_"solvent"`    ...(i)

For a binary mixture,

`chi_"solute" + chi_"solvent" = 1`

∴ `chi_"solvent" = 1 - chi_"solute"`

Putting this value in the equation (i), we have

`P = P^circ xx (1 - chi_"solute")`

or `P/P^circ = 1 - chi_"solute"`

or `1 - P/P^circ = chi_"solute"`

or `(P^circ - P)/P^circ = chi_"solute"`

In the above equation, the term (P° − P) represents the lowering of vapour pressure in the formation of the solution. The term `(P^circ - P)/P^circ`is called the relative lowering of vapour pressure of the solution.