The vapor pressure of benzene, c6h6, is 100.0 torr at 26.1 °c. assuming raoult's law is obeyed, how many moles of a nonvolatile solute must be added to 100.0 ml of benzene to decrease its vapor pressure by 10.0% at 26.1 °c? the density of benzene is 0.8765 g> cm3.

According to Raoult's law:

   -(1)

where is observed vapor pressure of the solution, is mole fraction of solvent, and is vapor pressure of the pure solvent.

Vapor pressure of benzene =   (given)

Reduction in vapor pressure on addition of non-volatile solute = 10.0 %   (given)

So, vapor pressure of the component in the solution =

Substituting the values in formula (1):

Mole of benzene =

Mole of benzene =

Since,

So,

Hence, moles of a nonvolatile solute to be added is .