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KVPY Sample Paper KVPY Stream-SX Model Paper-18

  • question_answer
    Standard enthalpy of formation of \[{{C}_{3}}{{H}_{7}}N{{O}_{2}}(g),\]\[C{{O}_{2}}(g)\] and \[{{H}_{2}}O\,(l)\] are \[-133.57,-94.05\] and \[-\,68.32\,kcal/mol\] respectively. Standard enthalpy of combustion of \[C{{H}_{4}}\] at \[25{}^\circ C\] is\[-\,212.8\,kcal/mol\]. Calculate the enthalpy for combustion in kcal/mol of \[{{C}_{3}}{{H}_{7}}N{{O}_{2}}(s).\]

    A) \[-\,372.69\]

    B) \[-\,387.70\]

    C) \[-\,370.89\]

    D) \[-\,372.16\]

    Correct Answer: B

    Solution :

    For the reaction, \[{{C}_{3}}{{H}_{7}}N{{O}_{2}}(s)+\frac{11}{2}{{O}_{2}}(g)\xrightarrow{{}}3C{{O}_{2}}(g)+\frac{1}{2}{{N}_{2}}(g)+\]\[\frac{7}{2}{{H}_{2}}O\,(g)\]
    \[\Delta {{H}_{comb}}=3\Delta {{H}_{f}}C{{O}_{2}}+0+\frac{7}{2}\Delta {{H}_{f}}({{H}_{2}}O)\]\[-\,\Delta {{H}_{f}}({{C}_{3}}{{H}_{7}}N{{O}_{2}})\,+\,0\]
    \[=3\,(-\,94.05)+\frac{7}{2}(-\,68.32)-(-\,133.57)\]\[=-\,387.70\,kcal/mol.\]


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