Question

How do you write an equation that shows the reaction of phosphate ions, `"PO"_4^(3-)"` reacting with hydronium ions, `"H"_3"O"^(+)"`?

Answer 1

`"H"_3"O"^+("aq")` + `"PO"_4""^(3-)("aq")` `rightleftharpoons` `"HPO"_4""^(2-)("aq")` + `"H"_2"O"("l")`

Notice that the total charge (2-) is the same on both sides of the equation, and that this is reaction can take place in both directions.