Acid dissociation constants for phosphoric acid are given below. A buffer with a pH = 7.4 can best be made by using

Question

Acid dissociation constants for phosphoric acid are given below.  A buffer with a pH = 7.4 can best be made by using

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To make a buffer with a pH of 7.4, we need a weak acid and its conjugate base with a pKa closest to 7.4. From the given pKa values of phosphoric acid, we can see that the closest pKa value is 7.21, which corresponds to the second dissociation step (H2PO4- ⇌ H+ + HPO42-). Therefore, we can use a mixture of NaH2PO4 and Na2HPO4 to make the buffer. Choice B is the only one that includes these two compounds. The addition of NaH2PO4 and Na2HPO4 (as well as NaHPO4) will increase the ionic strength of the buffer, but it will not significantly affect the pH. Choice A includes HS- and H2PO4-, but not the corresponding conjugate bases. Choice C includes NaHPO4 and Na3PO4, but not the corresponding weak acids. Choice D includes only NaH2PO4, which will not provide a sufficient buffer capacity. Choice E includes only Na2HPO4, which will not provide a sufficient buffer capacity either.

Acid Dissociation Constants for Phosphoric Acid Are Given Below