Equilibrium Constant Example Problem

This example problem demonstrates how to use the equilibrium constant of a reaction to determine equilibrium concentrations of reactants and products.

Problem:

For the reaction

H₂(g) + I₂(g) ↔ 2 HI(g)

the equilibrium constant is 7.1 x 10² at 25 °C. If the equilibrium concentration of H₂ is 0.81 M and the equilibrium concentration of I₂ is 0.035 M, what is the equilibrium concentration of HI?

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]

Solve for [HI]

[HI] = K[H₂][I₂]
[HI] = 7.1 x 10²(0.81 M)(0.035)
[HI] = 4.49 M

Answer:

The equilibrium concentration of [HI] is 4.49 M.