Analyze the reversible reaction to locate the product and reactants. The products are on the right side of the arrow, and the reactants are on the left side. For example, in the equation "Ca2+ + 2OH- <--> CaOH2," CaOH2 is the product and Ca2+ and OH- are the reactants.
Create an equilibrium equation by writing the concentrations of the products over the concentrations of the reactants. In this example, the equation is "K = [CaOH2]/ ([Ca2+][OH-])."
Raise the concentrations in the equation to the power of their respective coefficients in the reaction. In this example, the equation becomes "K = [CaOH2]/ ([Ca2+][OH-]^2)." This is the complete equilibrium equation.
Plug in the known values of concentration given in the problem and the K value to find any remaining concentration values. For example, if K is given as 120, [OH-] is .004 and [Ca2+] is .002, then [CaOH2] is 120*[.002]*[.004]^2, or .00000384.