Question

The normal boiling point of diethyl ether is 34.5 C. A solution containing a nonvolatile solute dissolved in diethyl ether has a vapor pressure of 698 torr at 34.5 C. What is the mole fraction of diethyl ether in this solution?

Answer 1

Total pressure of the system will be room temperature pressure which is 760 mmHg

Since normal boiling point of ether is 34.5 oC. the vapour pressure of diethyl ether must equal to that of the total pressure.

Hence Vapour pressure of diethyl ether = 760 mmHg

Since a non-volatile solute is now added .the vapour pressure of ether has went down

Vapour pressure of ether after addition of non volatile solute

We will apply roults law

YiP_t=P = Xi Psat

where P is partial pressure of the solution

Xi is the mole fraction in liquid state

Psat is the vapour pressure of the pure solvent.

In our case P = 698 Torr, Psat = 760 Torr

substituting this in the above equation,

698 = Xi*760

Xi =0.9184

Hence, mole fraction of diethyl in this solution is 0.9184