The vapour pressure of benzene at a certain temperature is 640 mm of Hg. When a nonvolatile and non-electrolyte solid weighing 2.175 g is added to 39.08 g of benzene, the vapour pressure of the solution becomes 600 mm of Hg. What is the molecular weight of solid substance?
(a) 69.60
(b) 49.59
(c) 59.60
(d) 108.30