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Question

The vapour pressure of pure benzene at certain temperature is 640 mm of Hg. A non-volatile non-electrolyte solid weighing 2.175 g is added to 39.0 g of benzene. The vapour pressure of the solution is 600 mm of Hg. What is the molar mass of the solid substance?

A
125 g
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B
105 g
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C
80.5 g
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D
65.25 g
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Solution

The correct option is D 65.25 gAccording to Raoult's law, P0−PsP0=nN Where, P0=Vapour pressure of benzne Ps=Vapour pressure of solutionn=number of moles of soluteN=number of moles of benzene wsolute×MsolventWsolvent×Msolute=P0−PsP0 Msolute=wsolute×MsolventWsolvent×P0P0−Ps wsolute=2.175 g Wbenzene=39.0 g Mbenzene=78 g/mol P0=640 mm Hg Ps=600 mm Hg Msolute=2.175×7839.0×600640−600 Msolute=2.175×7839.0×60040=65.25 g

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