But first, let's look at the reaction of an acid in an aqueous state with the water molecule in the liquid state. Well, what we get is a conjugate base in the acreage state and a conjugate acid in the aqueous states. So let's write the equilibrium constant expression for this reaction. So we get the acid ionization constant for this acid is equal to the concentration of hydronium times the concentration of the conjugate base. These two guys divided by the concentration of the conjugate acid for this guy. Now, if you are confused where this expression comes from, check out the link below.