What is the ratio of HPO4^2- to H2PO4^- at pH 6.2 if the pKa of H2PO4^- at 25 degrees Celsius is approximately 7.2?

To find the ratio of HPO4^2- to H2PO4^– at pH 6.2, we can use the Henderson-Hasselbalch equation, which is given by:

pH = pKa + log( [A^–]/[HA])

Where:

• pH = 6.2
• pKa = 7.2
• [A^–] = concentration of HPO4^2-
• [HA] = concentration of H2PO4^–

We can rearrange the equation to solve for the ratio:

log( [A^–]/[HA]) = pH – pKa

Plugging in the values:

log( [HPO4^2-]/[H2PO4^–]) = 6.2 – 7.2 = -1

To remove the logarithm, we exponentiate both sides:

[HPO4^2-]/[H2PO4^–] = 10^-1 = 0.1

This means that the ratio of HPO4^2- to H2PO4^– at pH 6.2 is 0.1:1. In other words, for every 1 part of H2PO4^–, there is 0.1 part of HPO4^2-.