Generally, the equilibrium constant (Keq) is the product of the concentration of the reaction products divided by the product of the concentration of the reaction reactants, each term is raised to a power equal to its coefficient in the balanced chemical reaction.
For the given reaction: 2HF → H₂ + F₂,
Keq = [H₂][F₂] / [HF]²,
Keq = [H₂][F₂] / [HF]² = [8.4 x 10⁻³][8.4 x 10⁻³] / [5.82 x 10⁻²]² = 2.1 x 10⁻².
