An Example of How To Find the Equilibrium Constant

This example problem demonstrates how to find the equilibrium constant of a reaction from equilibrium concentrations of reactants and products. Problem: For the reactionH2(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 MWhat is the equilibrium constant of this reaction? Solution The equilibrium constant (K) for the chemical equationaA bB ↔ cC dDcan be expressed by the concentrations of A,B,C and D at equilibrium by the equationK [C]c[D]d/[A]a[B]bFor this equation, there is no dD so it is left out of the equation.K [C]c/[A]a[B]bSubstitute for this reactionK [HI]2/[H2][I2]K (1.29 M)2/(0.106 M)(0.035 M)K 4.49 x 102 Answer: The equilibrium constant of this reaction is 4.49 x 102.