If $\mathrm{C}(s)+\mathrm{O}_{2}(g) \longrightarrow \mathrm{CO}_{2}(g) ; \Delta H=r$ and $\mathrm{CO}(g)+\frac{1}{2} \mathrm{O}_{2} \longrightarrow \mathrm{CO}_{2}(g): \Delta H=s$ then, the heat of formation of $\mathrm{CO}$ is

• A

  $r+s$
• Correct answer
  B

  $r-s$
• C

  $s-r$
• D

  $\mathrm{rs}$

Solution

$\mathrm{C}(s)+\mathrm{O}_{2}(g) \rightarrow \mathrm{CO}_{2}(g) ; \Delta H=r$ ...(i) $\mathrm{CO}(g)+\frac{1}{2} \mathrm{O}_{2}(g) \rightarrow \mathrm{CO}_{2}(g) ; \Delta H=s$ ...(ii) $\mathrm{C}(s)+\frac{1}{2} \mathrm{O}_{2}(g) \longrightarrow \mathrm{CO}(g) ; \Delta H=?$ Subtract Eq. (ii) from Eq. (i)

Hence, the correct answer is option: 'B'.