Iron (Fe) is obtained from rock that is extracted from open pit mines and then crushed. The process used to obtain the pure metal from the crushed rock produces solid waste, called tailings, which are stored in disposal areas near the mines. The tailings pose a serious environmental risk because they contain sulfides, such as pyrite , (FeS₂), which oxidize in air to produce metal ions and H⁺ ions that can enter into surface water or ground water. The oxidation of FeS₂ to Fe³⁺ is described by the unbalanced chemical expression below.

FeS₂(s) + O₂(g) + H₂O(l) → Fe³⁺(aq) + SO₄^2-(aq) + H⁺(aq)     (not balanced)

Thus, the oxidation of pyrite produces Fe³⁺ and H⁺ ions that can leach into surface or ground water. The leaching of H⁺ ions causes the water to become very acidic. To prevent acidification of nearby ground or surface water, limestone (CaCO₃) is added to the tailings to neutralize the H+ ions:

CaCO₃(s) + 2 H⁺(aq) → Ca²⁺(aq) + H₂O(l) + CO₂(g)

(a) Balance the equation above for the reaction of FeS₂ and O₂.

(b) What is the minimum amount of CaCO₃(s) required, per kilogram of tailings, to prevent contamination if the tailings contain 3% S by mass? Assume that all the sulfur in the tailings is in the form FeS₂.