Question

Phosphoric acid is a triprotic acid ( K a1 = 6.9 × 10 − 3 , K a2 = 6.2 × 10 − 8 , and K a3 = 4.8 × 10 − 13 ). To find the pH of a buffer composed of H 2 PO − 4 ( aq ) and HPO 2 − 4 ( aq ) , which p K a value should be used in the Henderson–Hasselbalch equation?

Answer 1

To calculate the pH of a buffer solution composed of H2PO−4(aq) and HPO2−4(aq), the specific pKa value for the ionization step of interest needs to be known.

Step-by-step explanation:

The Henderson-Hasselbalch equation is used to calculate the pH of a buffer solution.

In the case of phosphoric acid, which is a triprotic acid, we need to use the pKa value that corresponds to the ionization step we are interested in.

Since the question does not specify which buffer composition is being used, we cannot determine which pKa value to use from the information given.