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  4. The heat of combustion of solid benzoic acid at constant volume is -321.30kJ at 27oC . The heat of combustion at constant pressure is

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Q. The heat of combustion of solid benzoic acid at constant volume is $ -321.30kJ $ at $ {{27}^{o}}C $ . The heat of combustion at constant pressure is

BVP MedicalBVP Medical 2008

Solution:

$ {{C}_{6}}{{H}_{5}}COOH(s)+\frac{15}{2}{{O}_{2}}(g)\xrightarrow{{}} $ $ 7C{{O}_{2}}(g)+3{{H}_{2}}O(l) $ $ \Delta {{n}_{g}}={{n}_{p}}-{{n}_{r}}=7-\frac{15}{2}=\frac{-1}{2} $ $ {{q}_{p}}={{q}_{v}}+\Delta {{n}_{g}}(RT) $ $ =-321.30+\left( -\frac{1}{2} \right)300R $ $ =-321.30-150R $