How to Find the Equilibrium Constant of a Reaction

This example problem demonstrates how to find the equilibrium constant of a reaction from equilibrium concentrations of reactants and products.

For the reaction
H₂(g) + I₂(g) ↔ 2 HI(g)
At equilibrium, the concentrations are found to be
[H₂] = 0.106 M
[I₂] = 0.035 M
[HI] = 1.29 M
What is the equilibrium constant of this reaction?

The equilibrium constant (K) for the chemical equation
aA + bB ↔ cC + dD
can be expressed by the concentrations of A,B,C and D at equilibrium by the equation
K = [C]^c[D]^d/[A]^a[B]^b
For this equation, there is no dD so it is left out of the equation.
K = [C]^c/[A]^a[B]^b
Substitute for this reaction
K = [HI]²/[H₂][I₂]
K = (1.29 M)²/(0.106 M)(0.035 M)
K = 4.49 x 10²

The equilibrium constant of this reaction is 4.49 x 10².