What is the correct chemical formula for iron(III) phosphide?

• A. Fe2P3
• B. FePO4
• C. Fe3P2
• D. FeP

Answer: (C)

Verified step by step guidance

1. Identify the ions involved in the compound. Iron(III) indicates the iron ion has a charge of +3, written as Fe^{3+}. Phosphide is the ion formed by phosphorus with a charge of -3, written as P^{3-}.
2. Determine the ratio of ions needed to balance the overall charge of the compound. Since Fe^{3+} has a +3 charge and P^{3-} has a -3 charge, the total positive and negative charges must balance to zero.
3. Set up the charge balance equation: let the number of Fe^{3+} ions be x and the number of P^{3-} ions be y. The total charge is 3x + (-3)y = 0, which simplifies to 3x = 3y, or x = y.
4. Since the charges are equal in magnitude but opposite in sign, the simplest ratio of Fe^{3+} to P^{3-} ions is 1:1, leading to the empirical formula FeP.
5. However, the problem states the correct formula is Fe3P2, which suggests a different charge balance. Re-examine the charges: if Fe is +3 and P is -3, the formula Fe3P2 implies 3 Fe^{3+} ions and 2 P^{3-} ions, total charge: (3)(+3) + (2)(-3) = +9 - 6 = +3, which is not neutral. Therefore, the correct formula must balance charges exactly, so the correct formula is FeP. The problem's answer Fe3P2 corresponds to iron(II) phosphide, where Fe is +2 and P is -3. So, the correct approach is to confirm the oxidation states and balance charges accordingly.