NEET Sample Paper NEET Sample Test Paper-15

  • question_answer A gas expands isothermally against a constant external pressure of 1 atm from a volume of \[\text{10 d}{{\text{m}}^{\text{3}}}\]to a volume of \[\text{20 d}{{\text{m}}^{\text{3}}}\text{.}\] It absorbs 300 J of thermal energy from its surroundings. The \[\Delta U\]is

    A)  -312 J                        

    B)  -213 J

    C)  -321 J                        

    D)  -123 J

    Correct Answer: B

    Solution :

    \[w=--{{V}_{1}}\int_{{}}^{{}}{{{V}_{2}}.PdV=-P({{V}_{2}}-{{V}_{1}})}\] \[W=-(1atm)(20-10)=-10\,d{{m}^{3}}\,\text{atm}\text{.}\] \[=-10\,d{{m}^{3}}\times \frac{8.314\,J{{K}^{-1}}\,mo{{l}^{-1}}}{0.08206\,d{{m}^{3}}{{K}^{-1}}mo{{l}^{-1}}}\] \[=-1013\,J.\] According to the first law of thermodynamics \[\Delta U=q+w\] \[=800+(-1013=-213\,J)\] Hence, the correct option is [b].

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