How to Find the Equilibrium Constant of a Reaction

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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.
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    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 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?

    Solution

    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

    Answer:

    The equilibrium constant of this reaction is 4.49 x 102.