It is not surprising based on what we know about the nature of equilibrium, that there is a mathematical relationship between the concentration of the products and reactants of a reversible chemical reaction at a given temperature. The mathematical relationship is called the Law of Chemical Equilibrium. This PowerPoint presentation describes the format of the equilibrium expression and teaches students how to write one. It goes on to explain the meaning of the equilibrium constant with respect to the format of the equilibrium expression. Finally, it shows how to perform calculations using the equilibrium expression. Worksheets on this topic are available free of charge on my website at www.evanschemistrycorner.com.