Final answer:
According to the law of conservation of mass, two hydrogen atoms and two chlorine atoms are required on both reactant and product sides to balance the equation FeS + 2HCl → FeCl2 + H2S, complying with the conservation laws.
Step-by-step explanation:
According to the law of conservation of mass, the number of atoms of each element must be the same on the reactant side and product side in a chemical equation. In the reaction FeS + 2HCl → FeCl2 + H2S, each element should balance out.
Starting with hydrogen, there are two hydrogen atoms in 2 molecules of HCl on the reactant side, and similarly, we can see there are two hydrogen atoms on the product side in the form of H2S. Chlorine is present as two atoms within the 2 molecules of HCl and on the product side as two chlorine atoms within one molecule of FeCl2. This makes the reaction balanced with two hydrogen and two chlorine atoms on both sides of the equation. It satisfies the law of conservation of atoms and law of conservation of mass since the total mass of the reactants equals the total mass of the products.