[doodlingsamurai]

There's plenty of resources about how to make anhydrous FeCl3 from FeCl3.6H2O but say you have the anhydrous FeCl3, how would it become FeCl3.6H2O? Is exposure to air enough or would it need to be in contact with water?

[Borek]

Is exposure to air enough

Depends on how exactly ·6H₂O you need it. Most hydrates have rather variable composition, with exact stoichiometry (amount of water) depending on the partial pressure of water in the air.

But in general yes, if you leave the anhydrous FeCl₃ in the open for long enough, you will end up with something that is identical to FeCl₃·6H₂O left in the same place to equilibrate (exact final composition being a function of humidity).