The standard heat of formation $\left(\Delta_{f} \mathrm{H}_{298}^{0}\right)$ of ethane in $(\mathrm{kJ} / \mathrm{mol})$, if the heat of combustion of ethane, hydrogen and graphite are $-1560$, $-393.5$ and $-286 \mathrm{~kJ} / \mathrm{mol}$, respectively is
$2 \mathrm{C}($ graphite $)+3 \mathrm{H}_{2}(\mathrm{~g}) \longrightarrow \mathrm{C}_{2} \mathrm{H}_{6}(\mathrm{~g})$
$\Delta_{\mathrm{f}} \mathrm{H}\left(\mathrm{C}_{2} \mathrm{H}_{6}\right)=2 \Delta \mathrm{H}_{\text {comb }}\left(\mathrm{C}_{\text {gupphite }}\right)+3 \Delta \mathrm{H}_{\text {comb }}\left(\mathrm{H}_{2}\right)$
$-\Delta \mathrm{H}_{\mathrm{comb}}\left(\mathrm{C}_{2} \mathrm{H}_{6}\right)$
$=-(286 \times 2)-(393.5 \times 3)-(-1560)$
$=-572-1180.5+1560=-192.5 \mathrm{~kJ} /$ mole
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