Phosphate is derived from the titration of phosphoric acid, H3PO4. The three pKa’s of H3PO4 are 2.15, 7.20, and 12.35 respectively. 

a/ Write out the series of ionization (equilibrium) reactions corresponding to each ionization, making sure to write out the (flat) structure each molecule/ion as you do so. Mark correct chemical bonds.

b/ In your diagram above, circle the dominant form of phosphate as it would appear at pH 5.7.