The formula unit equation for the given reaction occurring in water is
CaS(aq) + FeBr₂(s) → FeS(s) + CaBr₂(aq).
<h3>What is chemical reaction?</h3>
Chemical reactions are those reaction in which reactant molecules react with each other for the formation of product.
When calcium sulfide reacts with iron (II) bromide for the formation of iron(II) sulfide and calcium bromide will be represented through the following equation:
CaS(aq) + FeBr₂(s) → FeS(s) + CaBr₂(aq)
Above reaction best explains the formula unit of given reaction as all valency of atoms are satisfied there.
Hence, the required formula unit is CaS(aq) + FeBr₂(s) → FeS(s) + CaBr₂(aq).
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