The vapour pressure of pure benzene at a certain temperature is 0.850 bar. A non-volatile, nonelectrolyte solid weighing (0.5 ~g ) is added to 39.0 ( g ) of benzene (molar mass (78 ~g / mol ) ). The vapour pressure of the solution then is 0.845 bar. What is the molecular mass of the solid substance?

Step-by-step solution

(P_A^ =0.850 ) bar, (P_S=0.845 ) bar (w=0.5 ~g , m= ? ) weight of solvent (benzene) (=39.0 ~g ) and molecular weight of benzene (=78 ~g ) As we know, ( P_A^ -P_S P_A^ =x_B= n_B n_A ) ( 0.850-0.845 0.850 = w_B M_B w_A M_A = 0.5 m 39 78 ) On solving, we get molecular mass of solid structure (( m )=170 ~g ).