【How-to】What is the percent dissociation

It is convenient to remember that any ionizing function is 50% unprotonated at a pH equal to its pKa, approximately 90% unprotonated (or protonated) at a pH unit one unit above (below) its pKa, and approximately 100%, actually 99.9%, unprotonated (protonated) at a pH 2 units above (below) its pK a.