pKa values of phosphoric acid are 2.2, 7.2 and 12.7. A phosphate buffer of pH = 7.4 can be prepared using Question 6 options: H3PO4 and HCl. H2PO4- and HPO42-. HPO42- and PO43-. None of the above

The correct answer is H2PO4- and HPO42-.

Here's why:

A buffer solution is most effective when the pH is close to the pKa of the acid. In this case, the pH of the buffer is 7.4. The pKa values of phosphoric acid are 2.2, 7.2, and 12.7. The pKa value closest to the pH of the buffer is 7.2.

At a pH of 7.2, phosphoric acid (H3PO4) loses two protons to become the dihydrogen phosphate ion (H2PO4-). As the pH increases slightly to 7.4, some of the H2PO4- will lose another proton to become the hydrogen phosphate ion (HPO42-).

Therefore, a buffer of pH 7.4 can be prepared using a mixture of H2PO4- and HPO42-.