Final answer:
The chemical formula for iron(III) phosphide is FeP, as the iron(III) ions (Fe3+) and the phosphide ions (P3-) have charges that balance each other out in a one-to-one ratio.
Step-by-step explanation:
The chemical formula for iron(III) phosphide is FeP. This compound is made up of iron (III) ions which have a +3 charge, and phosphide ions which have a -3 charge. As each ion has a charge that is equal in magnitude but opposite in sign, equal numbers of each ion will balance out the charges, thus only one of each is needed. Therefore, the chemical formula consists of one Fe3+ and one P3-, resulting in FeP.