- #1

chops369

- 56

- 0

## Homework Statement

If the total cellular concentration of phosphate is 20 mM (millimolar) and the pH is 7.4, the distribution of the major phosphate species is given by

pH = pKa + log

_{10}[HPO

_{4}

^{2-}] / [H

_{2}PO

_{4}

^{-}]

7.4 = 7.20 + log

_{10}[HPO

_{4}

^{2-}] / [H

_{2}PO

_{4}

^{-}]

[HPO

_{4}

^{2-}] / [H

_{2}PO

_{4}

^{-}] = 1.58

Thus, if [HPO

_{4}

^{2-}] + [H

_{2}PO

_{4}

^{-}] = 20 mM, then

[HPO

_{4}

^{2-}] = 12.25 mM and [H

_{2}PO

_{4}

^{-}] = 7.75 mM

## Homework Equations

pH = pKa + log

_{10}[A

^{-}] / [HA]

pH = -log

_{10}[H

^{+}]

## The Attempt at a Solution

I understand everything up until they provide the concentrations of each phosphate species. Since their ratio as shown in the equation is 1.58, one can clearly assume that [HPO

_{4}

^{2-}] > [H

_{2}PO

_{4}

^{-}]. But the fact that no explanation is provided for arriving at their specific concentrations is driving me insane.

The Henderson-Hasselbalch equation shows that, when [HPO

_{4}

^{2-}] / [H

_{2}PO

_{4}

^{-}] = 1, pH = pKa. But since we are at pH = 7.4, they obviously can't be equal. I think the solution must involve taking the 0.2 difference into account somehow.