The vapour pressure of benzene at a certain temperature is 640 mm of Hg. A non-volatile and non-electrolyte solid weighing 2.175g is added to 39.08g of benzene. If the vapour pressure of the solution is 600mm of Hg, what is the molecular weight of the solid substance?
a. 59.60 g/mol
b. 49.50 g/mol
c. 69.60 g/mol
d. 79.82 g/mol