JEE Main & Advanced Sample Paper JEE Main - Mock Test - 27

  • question_answer
    The fraction of molecules in one mole of an ideal gas that have energies greater than two times the average kinetic energy at \[50{}^\circ C\]and \[100{}^\circ C\] is

    A) \[2{{\left( \frac{6}{\pi } \right)}^{1/2}}\exp (-6)\]

    B)        \[{{\left( \frac{6}{\pi } \right)}^{1/2}}\exp (-6)\]

    C) \[2{{\left( \frac{3}{\pi } \right)}^{1/2}}\exp (-3)\]

    D)        \[{{\left( \frac{3}{\pi } \right)}^{1/2}}\exp (-3)\]

    Correct Answer: C

    Solution :

    [c] The average kinetic (or thermal energy) \[\overset{-}{\mathop{\varepsilon }}\,=\frac{3}{2}kT\] The fraction of molecules having energies greater than \[\varepsilon \]is given by: \[\frac{N(\varepsilon )}{{{N}_{A}}}=2{{\left( \frac{\varepsilon }{\pi kT} \right)}^{\frac{1}{4}}}\exp (-\varepsilon /kT)\] ?.(i) Now, \[=2\overset{-}{\mathop{\varepsilon }}\,=2\left( \frac{3}{2}kT \right)=3kT.\] Substituting this in equation (i), we have \[\left( \frac{N(\varepsilon )}{{{N}_{A}}} \right)=2{{\left( \frac{3kT}{\pi kT} \right)}^{\frac{1}{2}}}\exp (-3\,kT/kT)\] \[=2{{\left( \frac{3}{\pi } \right)}^{\frac{1}{2}}}\exp (-3)\]


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