Question

How do the differences in the polyatomic ions po3 3- and po4 3- help you determine whether each ends in -ite or -ate?

Answer 1

There is a rule in naming polyatomic anions containing oxygen. Those names ending in –ate are given to the most common oxyanion of an element. While those names ending in –ite are for those that contains fewer O atom for the same charge. Since P$PO_{3} ^{3-}$ has fewer O atoms, its name ends in –ite. Thus, P$PO_{3} ^{3-}$  is phosphite while P$PO_{4} ^{3-}$  is phosphate.