Which of the following species has the highest pKa value?
|D|| All the above species have almost the same pKa values, since they all have the same type of phosphate anionic counterpart|
The choices are phosphoric acid and its dissociation products (see above). In a polyprotic acid, such as H3PO4, almost invariably the first dissociation will be of the highest degree. So the highest pKa is for HPO42–. This is Choice C. For a comparison, the Ka values are given below. But as you know, we do not need them to answer this question.
H3PO4 ————— Ka1 = 7.5 x10– 3
H2PO4– ————— Ka2 = 6.2 x10– 8
HPO42– ————— Ka3 = 3.6 x10–11