posted by Joe Schmo .
Knowing that 2 aluminum atoms and 3 chlorine molecules react to produce 2 units of aluminum chloride, how many moles of aluminum chloride can be produced by reacting 7.8 moles of chlorine with excess aluminum?
I think it is clearer if you write an equation. 2Al + 3Cl2 ==> 2AlCl3
Now use the coefficients to convert moles Cl2 to moles AlCl3.