Phosphoric acid is a triprotic acid ( K a1 = 6.9 × 10 − 3 Ka1=6.9×10−3, K a2 = 6.2 × 10 − 8 Ka2=6.2×10−8, and K a3 = 4.8 × 10 − 13 Ka3=4.8×10−13). To find the pH of a buffer composed of H 2 PO − 4 ( aq ) H2PO4−(aq) and HPO 2 − 4 ( aq ) HPO42−(aq) , which p K a Ka value should be used in the Henderson–Hasselbalch equation?

To calculate the pH of the buffer composed of , we use the

Explanation:

Phosphoric acid is a triprotic acid and it will undergo three dissociation reaction each having their respective dissociation constants.

The chemical equation for the first dissociation reaction follows:

The chemical equation for the second dissociation reaction follows:

The chemical equation for the third dissociation reaction follows:

To form a buffer composed of , we use the of second dissociation process

To calculate the , we use the equation:

To calculate the pH of buffer, we use the equation given by Henderson Hasselbalch:

We are given:

= negative logarithm of second acid dissociation constant of phosphoric acid = 7.21

= concentration of conjugate base

= concentration of weak acid

Hence, to calculate the pH of the buffer composed of , we use the