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  4. The heat of combustion of solid benzoic acid at constant volume is -321.30 kJ at 27° C . 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 $ -321.30\,kJ $ at $ 27^{\circ} C $ . The heat of combustion at constant pressure is

Uttarkhand PMTUttarkhand PMT 2008

Solution:

$C _{6} H _{5} COOH (s)+\frac{15}{2} O _{2}(g) \rightarrow 7 CO _{2}(g)+3 H _{2} O (l)$
$\Delta n_{g}=n_{p}-n_{r}=7-\frac{15}{2}=\frac{-1}{2}$
$q_{p}=q_{v}+\Delta n_{g}(R T)$
$=-321.30+\left(-\frac{1}{2}\right) 300 R$
$=-321.30-150 \,R$