The intracellular fluid in muscle cells is maintained at a pH of $6.90$. Dihydrogen phosphate ions, $H_2{PO}_4^{-}$, and hydrogen phosphate ions, ${HPO}_4^{2-}$, act as a buffer to keep this pH stable. The $p{K}_a$ value for the acid dissociation of dihydrogen phosphate is $7.20$:

$$[H_2{PO}_4^{-}(aq)\rightleftharpoons H^{+}(aq)+{HPO}_4^{2-}(aq)$$

Explain, in terms of equilibrium, how this phosphate mixture acts as a buffer to control intracellular pH, and calculate the $[{HPO}_4^{2-}]:[H_2{PO}_4^{-}]$ ratio in this fluid.

Red blood cells contain hemoglobin, which contains ${Fe}^{2+}$ ions at the center of heme groups. Explain, using the concepts of coordination and ligand substitution:

• how hemoglobin binds and transports oxygen around the body,
• why carbon monoxide is highly toxic.