${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}\left(\mathrm{l}\right)\to 2{\mathrm{CO}}_2\left(\mathrm{g}\right)+3{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)++3{\mathrm{O}}_2\left(\mathrm{g}\right)$

Heat of combustion = Heat of formation of products - Heat of formation of reactants

Heat of combustion of hydrogen is equal to heat formation of water, and heat of combustion of carbon is equal to heat of formation of carbon dioxide.

$∆{\mathrm{H}}_{\mathrm{C}}=\left[2∆{{\mathrm{H}}_{\mathrm{f}}}^{\mathrm{o}}\left({\mathrm{CO}}_2\right)+3∆{{\mathrm{H}}_{\mathrm{f}}}^{\mathrm{o}}\left({\mathrm{H}}_2\mathrm{O}\right)\right]-\left[∆{{\mathrm{H}}_{\mathrm{f}}}^{\mathrm{o}}\left({\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}\left(\mathrm{l}\right)\right)\right]$

$–1234.7=[2\times (–393.5)+3\times (–241.8\left)\right]-\left[∆{{\mathrm{H}}_{\mathrm{f}}}^{\mathrm{o}}\left({\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}\left(\mathrm{l}\right)\right)\right]$

$∆{{\mathrm{H}}_{\mathrm{f}}}^{\mathrm{o}}{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}=-277.7 \mathrm{kJ}/\mathrm{mol}$

$\simeq -278 \mathrm{kJ}/\mathrm{mol}$