What is the formula for a barium chloride solution reacts with sodium sulfate solution to produce solid barium sulfate and sodium chloride in a solution?

The III is there because as a transition metal, iron

can have multiple oxidation states. This means iron is 3+. The

charge on each Cl is -1, so you need 3.

This gives you FeCl3

Next, Barium has 2+ charge, so you need two OH-'s

i.e. Ba(OH)2