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NEET AIPMT SOLVED PAPER 1999

  • 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?                                                             

    A)              49.50      

    B)              59.60       

    C)              69.40       

    D)              79.82

    Correct Answer: C

    Solution :

                    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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