What is the number of formula units in a 2.57 mole sample of FeCl3

Answer 1

You can think of a formula unit as a molecule. Thus, Avogadro's number applies to formula units just as it does to molecules, atoms, etc.

2.57 mol FeCl₃ x 6.02x10²³ form.units / mole = 1.55x10²⁴ formula units