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 Todd Helmenstine is a science writer and illustrator who has taught physics and math at the college level. He holds bachelor's degrees in both physics and mathematics. our editorial process 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.