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  • question_answer
    Vapour pressure of chloroform \[(CHC{{l}_{3}})\] and dichloromethane \[(C{{H}_{2}}C{{l}_{2}})\] at \[25{}^\circ C\] are 200 mmHg and 41.5 mmHg respectively. Vapour pressure of the solution obtained by mixing 25.5 g of \[CHC{{l}_{3}}\] and 40 g of \[C{{H}_{2}}C{{l}_{2}}\] at the same temperature will be (Molecular mass of \[CHC{{l}_{3}}=119.5\,u\] and molecular mass of \[C{{H}_{2}}C{{l}_{2}}=85\,u\])[AIPMT (M) 2012]

    A) 173.9 mmHg

    B) 615.0 mmHg

    C) 347.9 mmHg

    D) None of these

    Correct Answer: D

    Solution :

    [d] Number of moles of \[CHC{{l}_{3}},\]
                            \[{{n}_{A}}=\frac{w}{M}\]
                            \[=\frac{25.5}{119.5}\]
                            \[=0.213\]
                Number of moles of \[C{{H}_{2}}C{{l}_{2}},\]
                            \[{{n}_{B}}=\frac{40}{85}=0.47\]
                Mole fraction of \[CHC{{l}_{3}}\]
                            \[{{\chi }_{A}}=\frac{{{n}_{A}}}{{{n}_{A}}+{{n}_{B}}}\]
                            \[=\frac{0.213}{0.683}=0.31\]
                Mole fraction of \[C{{H}_{2}}C{{l}_{2}},\]
                            \[{{\chi }_{B}}=1-{{\chi }_{A}}\]
                            \[=1-0.31=0.69\]
                \[\because \] \[{{p}_{total}}={{p}_{A}}{{\chi }_{A}}+{{p}_{B}}{{\chi }_{B}}\]
                            \[=200\times 0.31+41.5\times 0.69\]
                            \[=62+28.63\]
                            \[=90.63mm\,Hg\]


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