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NEET Sample Paper NEET Sample Test Paper-59

  • question_answer
    How much energy is released when 6 moles of octane is burnt in air? Given \[\Delta H_{f}^{{}^\circ }\] for \[C{{O}_{2}}(g)\,,{{H}_{2}}O(g)\]and respectively are - 490, - 240 and+160kJ/mol:

    A) - 6.2kJ   

    B) - 37.4kJ

    C) - 35.5kJ          

    D) - 20.0kJ

    Correct Answer: B

    Solution :

    [b] ? 37.4kJ \[{{C}_{8}}{{H}_{18}}+\frac{25}{2}{{O}_{2}}\to 8C{{O}_{2}}+9{{H}_{2}}O\]             \[\Delta H{}^\circ \,reaction\,=[8\times \Delta H_{f}^{o}(C{{O}_{2}})+9\Delta H_{f}^{o}({{H}_{2}}O)]\]             \[-\left[ \Delta H_{f}^{o}({{C}_{8}}{{H}_{18}})+\frac{25}{2}\Delta H_{f}^{o}({{O}_{2}})] \right]\]             \[=\left[ 8\,(-490)+9(-240) \right]-\left[ (160)+\frac{25}{2}Co \right]\]             \[=-3920-2160-160=6420\,Jmo{{l}^{-1}}\]             \[=-6240\,J/mo{{l}^{-1}}\] \[\therefore \] For 6 moles, enthalpy of combustion, \[=-6240\times 6\] \[=-37400J=-3704KJ\]


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