For a particular isomer of \( \mathrm{C}_{8} \mathrm{H}_{18} \), the combustion reaction produces 5104.1 kJ of heat per mole of \( \mathrm{C}_{8} \mathrm{H}_{18}(\mathrm{~g}) \) consumed, under standard conditions.
\[
\mathrm{C}_{8} \mathrm{H}_{18}(\mathrm{~g})+\frac{25}{2} \mathrm{O}_{2}(\mathrm{~g}) \longrightarrow 8 \mathrm{CO}_{2}(\mathrm{~g})+9 \mathrm{H}_{2} \mathrm{O}(\mathrm{~g}) \quad \Delta H_{\mathrm{rxn}}^{\circ}=-5104.1 \mathrm{~kJ} / \mathrm{mol}
\]
What is the standard enthalpy of formation of this isomer of \( \mathrm{C}_{8} \mathrm{H}_{18}(\mathrm{~g}) \) ?
\[
\Delta H_{\mathrm{f}}^{\circ}=\square \mathrm{kJ} / \mathrm{mol}
\]
