Question

The vapor pressure of pure acetone is 266 torr. When a non-volatile solute is added, the vapor pressure of acetone above the solution falls to 214 torr. What is the mole fraction of the non-volatile solute in the solution?

Answer 1

Answer : The mole fraction of the non-volatile solute in the solution is 0.195

Explanation :

According to the relative lowering of vapor pressure, the vapor pressure of a component at a given temperature is equal to the mole fraction of that component of the solution multiplied by the vapor pressure of that component in the pure state.

Formula used :

$\frac{\Delta p}{p^o}=X_B$

$\frac{p^o-p_s}{p^o}=X_B$

where,

$p^o$ = vapor pressure of the pure solvent (acetone)  = 266 torr

$p_s$ = vapor pressure of the solution = 214 torr

$X_B$ = mole fraction of solute  = ?

Now put all the given values in the above formula, we get:

$\frac{p^o-p_s}{p^o}=X_B$

$\frac{266-214}{266}=X_B$

$X_B=0.195$

Therefore, the mole fraction of the non-volatile solute in the solution is 0.195