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  • question_answer
    The vapour pressure of benzene at a certain temperature is 640 mm. of Hg. A non-volatile and non-electrolyte solid, weighing 2.175 g is added to 39.08 g of benzene. If the vapour pressure of the solution is 600 mm of Hg, what is the molecular weight of solid substance? [AIPMT 1999]

    A) 49.50

    B) 59.60

    C) 69.40

    D) 79.82

    Correct Answer: C

    Solution :

    [c] According to Raoult’s law:
                            \[\frac{{{p}_{0}}-{{p}_{s}}}{{{p}_{0}}}=\frac{w\times M}{m\times W}\]
                            \[\frac{640-600}{640}=\frac{2.175\times 78}{m\times 39.08}\]
                            \[(M\,for\,{{C}_{6}}{{H}_{6}}=78)\]
                            \[m=\frac{2.175\times 78\times 640}{40\times 39.08}\]
                            \[=69.45\,\approx \,\,=69.4\]


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