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Question

# The vapour pressure of pure benzene at a certain temperature is 0.850 bar. A non-volatile, non-electrolyte solid weighing 0.5 g is added to 39.0 g of benzene (molar mass 78 g/mol). The vapour pressure of the solution then is 0.845 bar. What is the molecular mass of the solid substance?

A
58
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B
180
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C
170
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D
145
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Solution

## The correct option is C 170P∘benzene=0.850 bar;Wbenzene=39.0 g.M.W.=78 g/molP∘solution=0.845 bar;w=0.5 gLet n be the no. of moles of non volatile solid; w and m be the weight of solid and molecular mass respectively.Using, P∘benzene−P∘solutionP∘benzene=nsolidnbenzene0.850−0.8450.850=0.5m39780.0050.850=0.5m×7839m=100×0.8500.005=170

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