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

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.

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)?

The formula for Iron(III) sulfide is Fe2S3.

To understand how to derive this formula, we need to consider the charges of the iron (Fe) and sulfide (S) ions. Iron(III) indicates that the iron ion has a +3 charge, while sulfide carries a -2 charge.

To determine the formula, we need to balance out the charges of the ions. Since the sulfide ion has a charge of -2, we need two of these ions to match the charge of the iron(III) ion, which is +3. Therefore, we have Fe3+ and S2-.

Next, we cross the charges of the ions to obtain the subscripts for each element in the formula. The +3 charge of iron balances with three times the -2 charge of sulfide, resulting in the formula Fe2S3.

Essentially, the formula Fe2S3 indicates that the compound contains two iron ions, each with a +3 charge, and three sulfide ions, each with a -2 charge.