Here's how to determine the formula:
* Cobalt(III) indicates that cobalt has a +3 charge (Co3+).
* Iodide is the anion of iodine, which has a -1 charge (I-).
* To balance the charges, you need three iodide ions (3 x -1 = -3) to counter the +3 charge of the cobalt ion.
Therefore, the formula is CoI3.
