JEE Main & Advanced Sample Paper JEE Main - Mock Test - 36

  • question_answer
    For a gas containing \[{{10}^{23}}\] molecules (each having mass \[{{10}^{-22}}g\]) in a volume of \[1\text{ }d{{m}^{3}},\] the root mean square speed \[({{\mu }_{rms}})\] is \[{{10}^{5}}cm\,{{s}^{-1}}\]. Calculate the total kinetic energy of molecules and gas temperature. (Boltzmann constant, \[=1.38\times {{10}^{-23}}J{{K}^{-1}}\])

    A) \[0.5\times {{10}^{3}}J,\,241.5K\]    

    B) \[0.5\times {{10}^{4}}J,\,2415K\] 

    C) \[{{10}^{3}}J,\,483K,\]          

    D) \[{{10}^{4}}J,\,4830K\]

    Correct Answer: B

    Solution :

    [b] Mass \[-{{10}^{-22}}g={{10}^{-25}}kg,\] \[{{\mu }_{rms}}={{10}^{5}}cm\,\,{{s}^{-1}}={{10}^{3}}m{{s}^{-1}}\] Total kinetic energy             \[=N\left( \frac{1}{2}m{{u}^{2}} \right)\] \[={{10}^{23}}\left( \frac{1}{2}\times ({{10}^{-25}}kg){{({{10}^{3}}m\,\,{{s}^{-1}})}^{2}} \right)\] \[=0.5\times {{10}^{4}}kg\,\,{{m}^{2}}\,{{s}^{-2}}\] \[=0.5\times {{10}^{4}}J\] Total kinetic energy is also equal to \[\left( \frac{3}{2}NkT \right)\] Thus                          \[\frac{3}{2}NkT=0.5\times {{10}^{4}}J\] \[T=\frac{2}{3}\times \frac{0.5\times {{10}^{4}}J}{Nk}=\frac{2}{3}\times \frac{(0.5\times {{10}^{4}}J)}{({{10}^{23}})(1.38\times {{10}^{-23}}J\,{{K}^{-1}})}\]       \[=2415\,K\]  


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