If, at a given temperature, the equilibrium constant for the reaction H2 (g) Cl2 (g) 2HCl (g) is Kp, then the equilibrium constant for the reaction HCl (g) (1/2) H2 (g) (1/2) Cl2 (g) can be represented as:

Kp = [ (PH2/P°) ∧ (1/2) eq]*[ (PCl2/P°) ∧ (1/2) eq] / (PHCl (g) / P°) eq

Explanation:

H2 (g) + Cl2 (g) → 2HCl (g)

∴ Kp = F (T)

⇒ Kp = (PHCl/P°) eq / [ (PCl2/P°) eq]*[ (PH2/P°) eq]

∴ P° = 1 bar

HCl (g) → (1/2) H2 (g) + (1/2) Cl2 (g)

⇒ Kp = [ (PH2/P°) ∧ (1/2) eq]*[ (PCl2/P°) ∧ (1/2) eq] / (PHCl (g) / P°) eq