NEET Chemistry NEET PYQ-Solutions

  • question_answer
    A solution has a 1 : 4 mole ratio of pentane to hexane. The vapour pressure of the pure hydrocarbons at \[20{}^\circ C\] are 440 mm of Hg for pentane and 120 mm of Hg for hexane. The mole fraction of pentarie in the vapour phase would be: [AIPMT (S) 2005]

    A) 0.549

    B) 0.200   

    C) 0.786

    D) 0.478

    Correct Answer: D

    Solution :

    [d] Total vapour pressure of mixture
                = (Mole fraction of pentane V.P. of pentane) + (Mole fraction of hexane x V.P. of hexane) = V.P. of pentane in mixture + V.P. of hexane in mixture
                \[=\left( \frac{1}{5}\times 440+\frac{4}{5}\times 120 \right)=184\,mm\]
                \[\because \,\]     V.P. of pentane in mixture
                = V.P. of mixture x mole fraction of pentane it vapour phase
                88 = 184 x mole fraction of pentane in vapour phase
                \[\therefore \]      Mole fraction of pentane in vapour phase
                \[=\frac{88}{184}=0.478\]


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