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General Chemistry

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Acid-base Equilibrium

Question
5 out of 10
 

Which of the following species has the highest pKa value?

A H2PO4
B H3PO4
C HPO42 –
D All the above species have almost the same pKa values, since they all have the same type of phosphate anionic counterpart
Ans. C

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

Acid-base Equilibrium Flashcard List

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