Where does the 408 come from?

Question

2Al(s) + 3I₂(s) ==> 2AlI₃(s) ... balanced equation

Use the stoichiometry of the balanced equation along with dimensional analysis to find the theoretical yield:

18.3 g Al x 1 mol Al / 26.98 g x 2 mol AlI₃ / 2 mol Al x 408 g AlI₃/mol = 277 g AlI₃

Answer 1

The 408 has units of grams / mole and it is the molar mass of aluminum iodide (AlI₃)

Hope this clears things up for you.