The vapor pressure of pure benzene at a certain temperature is `640 mmHg`. A nonvolatile nonelectrolyte solid weighing `2.0 g` is added to `39 g` of benzene. The vapor pressure of the solution is `600 mmHg`. What is the molecular mass of the solid substance.
Strategy: Use Equation `(2.58)` as the concept of colligative properties is based on dilute solution. The moleculsr mass of benzene is `78u`.

Explanation

We are given
`P_(1)^(0)=640 mmHg, P=600 mmHg`
`w_(1)=39 g, w_(2)=2.0 g`
`M_(1)=78 g mol^(-1), M_(2)=M`
where M is the molar mass of solute,
According to Equation `(2.58)`, we have
`(P_(1)^(0)_P)/P_(1)^(0)=(w_(2)xxM_(1))/(w_(1)xxM_(1))`
Thus `((640-600) mmHg)/(640 mmHg)=((2.0 g)(78 g mol^(-1)))/((39 g)(M g mol^(-1)))`
`0.0625=4/M`
or `M=64 g mol^(-1)`
Thus, molecular mass of nonvolatile solute is `64 u`.