Question

What is the charge on the iron ion in Fe2S3?

Answer 1

Iron(III) sulfide is the name of the compound written using the Stock system. In the Stock system (named for Alfred Stock) a Roman numeral is used to indicate the oxidation number of the metal. Roman numerals are used in the names of compounds where the metal exhibits two or more oxidation states. 

The sum of the oxidation states of a compound must always be zero. The oxidation state of sulfide is -2, and the oxidation state of sulfur, we are told by (III) is +3. You can either use the "criss-cross" trick to get the subscripts where the oxidation number of Fe becomes the subscript of S, and the oxidation number of S becomes the subscript of Fe... 
+3-2 
.FeS --> Fe2S3 .

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