Final answer:
In a phosphate buffer at pH 7.4, the primary components are dihydrogen phosphate (H2PO4-) and hydrogen phosphate (HPO42-) due to their equilibrium near physiological pH.
Step-by-step explanation:
The primary chemical components present in phosphate buffer at pH 7.4 are H2PO4- (dihydrogen phosphate) and HPO42- (hydrogen phosphate). At this pH, which is close to physiological pH, these components are in equilibrium, according to the Henderson-Hasselbalch equation. The relevant pKa value to consider for a buffer at pH 7.4 is the one closest to this pH, which for phosphate is 6.86, corresponding to the HPO42- / PO43- system.
However, since the buffer pH of 7.4 is close to this pKa value, both H2PO4- and HPO42- ions will be present in significant amounts. Therefore the correct answer is A) H2PO4- and HPO42-.