Science, Tech, Math › Science How to Find the Equilibrium Constant of a Reaction Share Flipboard Email Print Getty Images/Yagi Studio Science Chemistry Basics Chemical Laws Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate by Todd Helmenstine Updated January 02, 2019 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. Continue Reading