Question

The normal boiling point of diethyl ether is 34.5 degrees C. A solution containing a nonvolatile solute dissolved in diethyl ether has a vapor pressure of 675 torr at 34.5 degrees C. What is the mole fraction of diethyl ether in the solution?

Answer 1

Solution-

Total pressure of the system ( room temperature pressure)= 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

Now when 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

Here now we apply roults law

YiPt=P = Xi Psat

here P= partial pressure of the solution

Xi = the mole fraction in liquid state

Psat is the vapour pressure of the pure solvent.

From the question
P = 675 Torr, Psat = 760 Torr

substituting this in the above equation,

675 = Xi*760

Xi =0.89

Therefore, mole fraction of diethyl = 0.89