The vapour pressure of pure benzene at a certain temperature is 0.850 bar. A non - volatile, non - electrolyte solid weighing 0.5 g 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 ?

The vapour pressure of pure benzene at a certain temperature is 0.850

1.	The vapour pressure of pure benzene at a certain temperature is 0.850 bar. A non - volatile, non - electrolyte solid weighing 0.5 g 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 ?
Answer» <P> Solution :The various quantities known to US are as follows: `p_(1)^(0)=0.850 " bar," p_(1)=0.845`bar `M_(1)=78.0 g mol^(-1), w_(2) =0.5 g, w_(1)=39 g` Substituting these VALUES in equation : `(p_(1)^(0)-p_(1))/(p_(1)^(0))=(w_(2)xx M_(1))/(M_(2)xx w_(1))` `(0.850 " bar " - 0.845 " bar")/("0.850 bar")=(0.5 g xx 78 g mol^(-1))/(M_(2)xx 39 g)` THEREFORE, `M_(2)=170 g mol^(-1)`.