In which one of the following equilibria, KP KC?

Condition: $K_P \neq K_C$ when $\Delta n_g \neq 0$

Step 1 — Calculate $\Delta n_g$ for each:

(1) $2\mathrm{HI} \rightleftharpoons \mathrm{H_2 + I_2}$: $\Delta n_g = 2 - 2 = 0 \Rightarrow K_P = K_C$ ✗

(2) $2\mathrm{C(s) + O_2(g)} \rightleftharpoons 2\mathrm{CO(g)}$: Only gases count. $\Delta n_g = 2 - 1 = 1 \neq 0 \Rightarrow K_P \neq K_C$ ✓

(3) $2\mathrm{NO} \rightleftharpoons \mathrm{N_2 + O_2}$: $\Delta n_g = 2 - 2 = 0 \Rightarrow K_P = K_C$ ✗

(4) $\mathrm{NO_2 + SO_2} \rightleftharpoons \mathrm{NO + SO_3}$: $\Delta n_g = 2 - 2 = 0 \Rightarrow K_P = K_C$ ✗

Answer: Option (2)