Acid dissociation constants for phosphoric acid are given below. $\begin{array}{|l|l|l|l|}
\hline & \mathrm{K}_{\mathrm{a} 1} & \mathrm{~K}_{\mathrm{a} 2} & \mathrm{~K}_{\mathrm{a} 3} \
\hline \mathrm{H}_{3} \mathrm{PO}_{4} & 7.5 \times 10^{-3} & 6.2 \times 10^{-8} & 4.8 \times 10^{-13} \
\hline
\end{array}$ A buffer with a pH = 7.4 can best be made by using

Question

Quizplus · Accepted Answer

To make a buffer with pH = 7.4, we want the solution to be close to the pKa of one of the species in the buffer. Looking at the given acid dissociation constants, we see that phosphoric acid has three pKa values: 2.15, 7.20, and 12.35. A pH of 7.4 is closest to the pKa2 value of 7.20. To create a buffer with the desired pH of 7.4, we need to choose a weak acid and its conjugate base with pKa values close to 7.20. 
Option B contains NaHS and Na2HPO4 which are weak acid and conjugate base, respectively, with relevant pKa values of 6.70 and 7.24 that are close to 7.20, making it the best option to make a buffer with pH = 7.4.

Acid Dissociation Constants for Phosphoric Acid Are Given Below A Buffer with a PH = 7