Final answer:
The coefficient of Fe, when the formula equation Fe₃O₄ + Al → Al₂O₃ + Fe is correctly balanced, is 9.
Step-by-step explanation:
When balancing the formula equation Fe₃O₄ + Al → Al₂O₃ + Fe, the correct coefficients must be added to ensure that the number of atoms of each element is the same on both sides of the equation. The balanced equation is 3Fe₃O₄ + 8Al → 4Al₂O₃ + 9Fe. Therefore, the correct coefficient of Fe, when the equation is balanced, is 9.
The chemical reaction in question is a single replacement reaction, and when balanced, the coefficients accurately reflect that the iron and aluminum atoms are conserved in the reaction. This conservation is key to balancing any chemical equation.