Question

predict how many sodium ions would combine with:
(a) a chlorine ion.
(b) a carbonate ion.
(c) a phosphate ion.

Answer 1

You need to know the valences of the Na^+ (which is +1) and the valences of the anions. Chloride ion is -1. The others are listed at this site.
http://www.woodrow.org/teachers/chemistry/links/chem1/PolyatomicIonsChart.html