What is the formula for Iron(III) sulfide? And why?

Answer 1

I think the answer is Fe_2S_3 (underscore meaning subscripts). But not sure how the numbers are associated. And why is Iron (II) sulfide = FeS (no subscript)?

Answer 2

iron(III) sulfide = iron(III) + S(II) so formula is, and you are correct, Fe2S3. If iron is II and S is II, it should be obvious that FeS is iron(II) sulfide.