Final answer:
The correct formula for the precipitate when FeCl3 and (NH4)2S are mixed is FeS (iron(II) sulfide).
Step-by-step explanation:
When aqueous solutions of FeCl3 and (NH4)2S are mixed, a solid precipitate forms due to the reaction between Fe3+ and S2- ions.
According to the solubility rules for sulfides, and considering that Fe3+ will be reduced to Fe2+ in the presence of sulfide (S2-), the most likely precipitate is FeS (iron(II) sulfide). The balanced precipitation reaction can be written as:
2 FeCl3(aq) + 3 (NH4)2S(aq) → 6 NH4Cl(aq) + 2 FeS(s)