For both cases, adjusting the pH alters the proportions of the acid form and of the conjugate base. The graph plots the concentration of the free acid AH (green curve) and the ionized conjugate base A− (red curve) as percentages of the total concentration as the pH is varied. At low pH the compound exists entirely as AH and at high pH entirely as A−. At the pKa the con centration of each species, AH and A−, is the same. At pHs near the pKa the compound exists as a mixture of the two forms.

Now we have established why you need to understand acids and bases; we must move on to consider why some acids are stronger than other acids and some bases stronger than other bases. To do this we must be able to estimate the pKa of common classes of organic compounds.

You do not need to learn exact figures for pKa values, but you will certainly need to develop a feel for approximate values—we will guide you towards which figures are worth learning and which you can leave to be looked up when you need them.