The vapour pressure of benzene at a certain temperature is 640 mm of Hg. A non-volatile and non-electrolyte solid weighing 2.175 g is added to 39.08 g of benzene. If the vapour pressure of the solution is 600 mm 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