JEE Main & Advanced Physics Kinetic Theory of Gases JEE PYQ-Kinetic Theory Of Gases

  • question_answer
    The temperature of an open room of volume \[30{{m}^{3}}\]increases from \[17{{\,}^{o}}C\]to\[27{{\,}^{o}}C\] due to sunshine. The atmospheric pressure in the room remains \[1\times {{10}^{5}}\,Pa.\] If \[{{n}_{i}}\]and \[{{n}_{f}}\] are the number of molecules in the room before and after heating, then \[{{n}_{f}}-{{n}_{i}}\]will be:-[JEE Main 2017]

    A)  \[2.5\times {{10}^{25}}\]

    B)                  \[-2.5\times {{10}^{25}}\]

    C)  \[-1.61\times {{10}^{23}}\]

    D)       \[1.38\times {{10}^{23}}\]

    Correct Answer: B

    Solution :

    [b] Using ideal gas equation
    PV = NRT
    (N is number of moles)
    \[{{P}_{0}}{{V}_{0}}={{N}_{i}}R\times 290\]                         …..[a]
    \[[{{T}_{i}}=237+17=290K]\]
    After heating
    \[{{P}_{0}}{{V}_{0}}={{N}_{f}}R\times 300\]                         …..[b]
    from equation [a] and [b]
    \[{{N}_{f}}-{{N}_{i}}=\frac{{{P}_{0}}{{V}_{0}}}{R}\left[ \frac{10}{290\times 300} \right]\]
    Hence \[{{n}_{f}}-{{n}_{i}}\]is
    \[=-\frac{{{P}_{0}}{{V}_{0}}}{R}\times \left[ \frac{10}{290\times 300} \right]\times 6.023\times {{10}^{23}}\]
    Putting \[{{P}_{0}}={{10}^{5}}{{P}_{A}}\]and \[{{V}_{0}}=30{{m}^{3}}\]
    Number of molecules \[{{n}_{f}}-{{n}_{i}}=-2.5\times {{10}^{25}}\]


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