Chemical equilibrium refers to a state in which the products of a reaction are being converted back to reactants at the same rate that the reactants are being converted to products. You can calculate the equilibrium constant, K, using a formula. You can then use this constant to calculate the concentration of a particular reactant at equilibrium.
Things You'll Need
 Balanced chemical equation
 Concentration of reactants or equilibrium constant
 Calculator
Finding the Equilibrium Constant

Write the balanced chemical equation on your paper. Circle the coefficients. For example, in the equation:
N2 + 3H2 <> 2NH3, circle the 3 and the 2.

Write the formula for finding the equilibrium constant using the circled coefficients. The general formula is:
K = [product 1]^coefficient [product 2]^coefficient / [reactant 1]^coefficient [reactant 2]^coefficient.
[] is shorthand for "concentration of."
For N2 + 3H2 <> 2NH3, given concentrations of 0.875 M, 0.598 M, and 0.105 M respectively, the formula for the equilibrium coefficient is:
[0.105 M]^2/[0.875 M][0.598 M]^3

Calculate the equilibrium constant.
Finding Equilibrium Concentrations

Write the equation for the balanced chemical reaction. Write all known equilibrium concentrations for the reaction underneath the relevant products. For example, use the equation N2 + 3H2 <> 2NH3 with known concentrations at equilibrium for N2 of 0.231 M and H2 of 0.629 M. Write the chemical equation:
N2 + 3H2 <> 2NH3.
Underneath N2, write 0.231 M. Underneath H2 write 0.629 M.

Write the equilibrium constant equation and rearrange it to solve for the missing concentration. You need the value of the equilibrium constant. For N2 + 3H2 <> 2NH3, K = 0.0589.
0.0589 = [NH3]^2/[0.231 M] * [0.629 M]^3.
Rearrange to solve for [NH3]:
0.0589 0.231 M (0.629M)^3 = [NH3]^2.

Take the square root of both sides to find [NH3].
References
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