Balanced chemical reaction: 2H₂(g) + O₂(g) ⇄ 2H₂O(g).
K = [H₂O]² / [O₂] · [H₂]².
[H₂O] is equilibrium concentration of water.
[O₂] is equilibrium concentration of molecule of oxygen.
[H₂] is equilibrium concentration of molecule of molecule of hydrogen.
The substances on the right side of the reaction are written at the top of the equilibrium constant expression and those on the left at the bottom.