Final answer:
The compound iron(III) phosphide is composed of iron(III) ions (Fe3+) and phosphide ions (P3-) in a 1:1 ratio, resulting in the formula FeP.
Step-by-step explanation:
The formula for the compound iron(III) phosphide can be derived by balancing the charges of the iron(III) ion, which has a charge of +3 (Fe3+), and the phosphide ion, which has a charge of -3 (P3-). Since each iron ion has a charge of +3 and each phosphide ion has a charge of -3, only one of each ion is needed to balance out the charges. The formula of the compound is FeP, and it is named iron(III) phosphide.