What is the gas produced when iron (II) sulfide is treated with a nonoxidizing acid?

Hydrogen sulfide

Explanation:

A nonoxidizing acid is the acid which cannot act as oxidizing agent and thus furnish hydrogen ions. Example: HCl

Iron (II) sulfide reacts with nonoxidizing acid, say hydrochloric acid to give rotten egg smelling gas, H₂S (Hydrogen sulfide).

The reaction is shown below:

FeS (s) + 2HCl (aq) ⇒ FeCl₂ (s) + H₂S (g)

A general reaction is also shown below of the reaction of Iron (II) sulfide with any nonoxidizing acid, HX as:

FeS (s) + 2HX (aq) ⇒ FeX₂ (s) + H₂S (g)