How to Find the Equilibrium Constant of a Reaction

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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.