JEE Main & Advanced Physics Kinetic Theory of Gases Sample Paper Topic Test - Kinetic Theory of Gases

  • question_answer
    A container has \[{{n}_{1}}\] moles of a monoatomic gas and \[{{n}_{2}}\] moles of a diatomic gas. The molar specific heat capacity at constant volume \[({{C}_{v}})\] of the mixture is found to be 2R. Then the ratio \[{{n}_{1}}/{{n}_{2}}\] is

    A) 3/5

    B) 5/3

    C) 1

    D)  none of these

    Correct Answer: C

    Solution :

    [c]: Average number of degree of freedom per molecule
    \[f=\frac{\text{Total number of degree of freedom}}{\text{Total number of molecule}}\]
    \[=\frac{{{n}_{1}}{{N}_{A}}{{f}_{1}}+{{n}_{2}}{{N}_{A}}{{f}_{2}}}{{{n}_{1}}{{N}_{A}}+{{n}_{2}}{{N}_{A}}}=\frac{{{n}_{1}}{{f}_{1}}+{{n}_{2}}{{f}_{2}}}{{{n}_{1}}+{{n}_{2}}}\]           
    Here, \[{{f}_{1}}=3,\,{{f}_{2}}=5\] and \[\gamma =1+1/f\]
    Also, \[\gamma =\frac{{{C}_{p}}}{{{C}_{v}}}=\frac{3R}{2R}\,\,=1.5=1+1\,/f\,\Rightarrow \,\,f=4\]
    Hence, \[\frac{3{{n}_{1}}+5{{n}_{2}}}{{{n}_{1}}+{{n}_{2}}}=4\] which gives \[{{n}_{1}}/{{n}_{2}}=1\]


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