NEET Sample Paper NEET Sample Test Paper-16

  • question_answer A gas expands isothermally against a constant external pressure of I atmosphere from a volume of \[10\text{ }d{{m}^{3}}\]to a volume of \[20\text{ }d{{m}^{3}}.\]In this process it absorbs 800 J of thermal energy from its surroundings. The \[\Delta U\]for the process in Joules is

    A)  -113 J                        

    B)  +113 J          

    C)                   -213 J             

    D)       +313 J

    Correct Answer: C

    Solution :

    We know that\[W=-\int\limits_{{{V}_{1}}}^{{{V}_{2}}}{PdV}\] Here, P = constant. So, \[W=-P({{V}_{2}}-{{V}_{1}})\] \[W=-(1)(20-10)=-10\,d{{m}^{3}}\,\text{atm}\] \[=-10\,d{{m}^{3}}\times \frac{8.314\,J{{K}^{-1}}\,mo{{l}^{-1}}}{0.08206\,d{{m}^{3}}\,mo{{l}^{-1}}}\] \[W=-1013\,J.\] Now using the first law of thermodynamics, \[\Delta U=Q+W=80\,J+(-1013)=-213\,J.\] Hence, the correct option is (c).

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