An aqueous solution of lead II phosphate is added to an aqueous solution of potassium iodide. When the two colorless solutions mix, a bright yellow precipitate forms in a double replacement reaction. What is the balanced chemical equation?

##PbHPO_4(aq) + 2KI(aq) rarr PbI_2(s)darr + KH_2PO_4##

This question is unfortunate as phosphate salts tend to be as soluble as bricks, and lead phosphate, ##Pb_3(PO_4)_2## would be even more insoluble. Thus I have assumed a biphosphate salt; ##HPO_4^(2-)## can have some solubility (at any rate phosphate ion, ##PO_4^(3-)##, is fairly rare, and speciates to ##HPO_4^(2-)## in aqueous solution).