Acid-Base Equilibrium Ka and Kb

Strong acids completely dissociate (ionize) in water. Weak acids partially dissociate and establish an equilibrium system. There is a large range of weak acids based upon their ability to donate protons. Consider the general weak acid HA and its reaction when placed in water  

The HA is not necessarily neutral, and the A does not necessarily have a negative charge. 

We can write an equilibrium constant expression for this system  

The [H20] is assumed to be a constant and is incorporated into the Ka value. It does not appear in the equilibrium constant expression. 

Since this is the equilibrium constant associated with a weak acid dissociation, this particular Kc is the weak add dissociation constant, Ka. The IQ expression is  

---