Write the formulas of the ions (including charges) present in aqueous solutions of the following:
K2CO3
NH4NO3
NA2Cr2O7
Mg(C2H3O2)2
Make sure it is both mass and charge balanced.
Here is a listing of many polyatomic ions. This should give you a start. Post with specific questions but tell us what you don't understand about it.
(Broken Link Removed)