The vapour pressure of pure benzene at a certain temperature is 0.850 bar. A non-volatile, non-electrolyte solid weighing 0.5g when added to 39.0 g of benzene (molar mass 78 g `"mol"^(-1)`), vapour pressure of the solution, then, is 0.845 bar. What is the molar mass of the solid substance ?

Explanation

The various quantities known to us are as follows:
`p_(1)^(0) = 0.850 ` bar , p = 0.845 bar , `m_(1)= 78 " g mol"^(-1) omega_(2) = 0.5 g , omega_(1) = 39 ` g
Substituting these values in equation (2.28), we get
` (0.850 "bar" - 0.845 "bar")/(0.850 "bar") = (0.5 g xx 78 g mol^(-1))/(M_(2)xx39 g) `
Therefore , `M_(2) = 170 " g mol"^(-1)`