Chemistry - Thermochemistry Question with Solution | TestHub

$$\begin{array}{rl}& :\mathrm{C}+{\mathrm{O}}_2\to {\mathrm{CO}}_2;\Delta H^{\circ }=-a\mathrm{kJ}\\ & 2\mathrm{CO}+{\mathrm{O}}_2\to 2{\mathrm{CO}}_2;\Delta H^{\circ }=-b\mathrm{kJ}\end{array}$$ Formation of $\mathrm{CO}$ can be written as : $$\mathrm{C}+\frac{1}{2}{\mathrm{O}}_2\to \mathrm{CO};\Delta H^{\circ }=?$$ Adding equation: $\mathrm{C}+{\mathrm{O}}_2\to {\mathrm{CO}}_2;\Delta H^{\circ }=-a\mathrm{kJ}$ $${\mathrm{CO}}_2\to \mathrm{CO}+\frac{1}{2}{\mathrm{O}}_2;\Delta H^{\circ }=\frac{b}{2}\mathrm{kJ}$$ We get $$\Delta H_f^{\circ }(\mathrm{CO})=\frac{b}{2}-a=\frac{b-2a}{2}$$