Write down the chemical reaction for which you want to find the equilibrium constant. This article will consider the Haber Process, which the Germans used in World War I to synthesize explosives. The equation for the Haber Process is N2+H2 --> NH3. Remember, the number that comes before a compound is its coefficient.
Balance the reaction so that the number of atoms of each element is the same on both sides of the equation. This usually involves putting coefficients in front of some of the elements or compounds in the equation. In this case, the balanced equation, with all coefficients in place, would be N2+3H2 --> 2NH3. Notice that there are two nitrogen atoms and six hydrogen atoms on both sides of the equation.
Write down or find the concentration of each of the products and each of the reactants. Reactants are the chemicals that you start with on the left side of the equation, and products are the chemicals that you end up with on the right side of the equation. In this case, we will assume a 1 Molar (M) concentration of N2, a 3M concentration of H2, and a 2M concentration of NH3.
Raise the concentration of each product or reactant to the power of its coefficient. For example, H2 has a coefficient of 3, so you would cube the concentration of H2 (3 molar) to get 27. Nitrogen gas in this equation has a coefficient of 1, so you would simply raise 1M to the first power and get 1. The concentration of NH3, the end product, has a coefficient of 2, so you would square 2M to get 4.
Multiply all of the product values by each other and all of the reactant values by each other to get one final product value and one final reactant value. Keep these two values separate. In this example, nitrogen and hydrogen are the reactants, so you would multiply 27 by 1 to get a final reactant value of 27. There is only one product, so your final product value is 4.
Divide your final product value by your final reactant value to get the equilibrium constant Kc. In this example, you would divide 4 by 27 to get 0.148, which is the final value of your equilibrium constant.