Chemistry - Thermochemistry Question with Solution | TestHub

$$\begin{array}{r}\mathrm{C}+{\mathrm{O}}_2\to {\mathrm{CO}}_2;\Delta H_f^{\circ }=-490\mathrm{kJ}/\mathrm{mol}\times 8\\ {\mathrm{H}}_2+\frac{1}{2}{\mathrm{O}}_2\to {\mathrm{H}}_2\mathrm{O};\Delta H_f^{\circ }=-240\mathrm{kJ}/\mathrm{mol}\times 9\\ 8\mathrm{C}+18\mathrm{H}\to {\mathrm{C}}_8{\mathrm{H}}_{18};\Delta H_f^{\circ }=+160\mathrm{kJ}/\mathrm{mol}\\ 8\mathrm{C}+8{\mathrm{O}}_2+9{\mathrm{H}}_2+\frac{9}{2}{\mathrm{O}}_2-8\mathrm{C}-18\mathrm{H}\\ \to 8{\mathrm{CO}}_2+9{\mathrm{H}}_2\mathrm{O}-{\mathrm{C}}_8{\mathrm{H}}_{18}\\ \Delta H_f^{\circ }=-3920-2160-160\\ {\mathrm{C}}_8{\mathrm{H}}_{18}+\frac{25}{2}{\mathrm{O}}_2\to 8{\mathrm{CO}}_2+9{\mathrm{H}}_2\mathrm{O};\Delta H^{\circ }=-6240\mathrm{kJ}/\mathrm{mol}\\ \Delta H^{\circ }\text{ for }6\mathrm{moles}\text{ of octane }=-6240\times 6=-37440\mathrm{kJ}/\mathrm{mol}.\end{array}$$