K – The Equilibrium Constant Expression

Click for a description of the historical experiment
Click to see how Guldberg and Waage determined the "law of mass action"

In 1864 two Norwegian chemists, Peter Waage and Cato Guldberg showed empirically, that for a chemical reaction at equilibrium, there is a mathematical expression that has a constant value, provided the temperature does not change.  This is called Keq (where the eq subscript represents the fact that this is an equilibrium system).  Because the equilibrium constant expression is so widely used in chemistry, it is often just written as K.  At other times, the subscript is changed to represent a particular type of equilbrium.  For example, you will see the following K's:

Ksp solubility product constant
Kw water dissociation constant
Ka acid dissociation constant
Kc concentration equilibrium constant
Kp pressure equilibrium constant

So, what is this K expression?  Let's develop it "experimentally" by using a hydrogen iodide simulation.