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BCECE Medical BCECE Medical Solved Papers-2001

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

    A)  20            

    B)  40

    C)  60             

    D)  80

    Correct Answer: D

    Solution :

    Key Idea: Use Raoults law to find molecular mass of solute. Given, \[{{p}^{o}}=\] Vapour pressure \[{{C}_{6}}{{H}_{6}}\] ^H^                 \[=200\,\,mm\,\,Hg\] p = vapour pressure of solution = 195 mm Hg W = weight of solvent \[({{C}_{6}}{{H}_{6}})=78\,\,g\] M = molecular wt. of \[{{C}_{6}}{{H}_{6}}=78\] w = weight of solute = 2 g m = molecular wt. of solute = ?                 \[\frac{{{p}^{o}}-p}{{{p}^{o}}}=\frac{w\times M}{m\times W}\] or            \[\frac{200-195}{200}=\frac{2\times 78}{m\times 78}\]                 or \[\frac{5}{200}=\frac{2}{m}\]  \[\therefore \] \[m=80\]


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