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Problem on molecular weight of solid

The vapor pressure of pure benzene at a certain temperature is 200 mm Hg. At the same temperature the vapor pressure of a solution containing 2g of non-volatile non-electrolyte solid in 78g of benzene is 195 mm Hg. What is the molecular weight of solid:

(a) 50 (b) 70 (c) 85  (d) 80

Answer (d): (Po - Ps)/Po = n/(n + N)
(Po - Ps)/Po = (w x M)/ (m x W) = 80

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