How many moles of O2 are needed to combine with 0.646 mol Al to give Al2O3?
Write the equation and balance it.
4Al + 3O2 ==> 2Al2O3
You have moles Al. Using the coefficients in the balanced equation, convert moles Al to moles Al2O3.
0.646 mol Al x (2 moles Al2O3/4 moles Al) = ?? moles Al2O3.
Note how the unit you don't want cancels (you want to get rid of moles Al and this does that) but it leaves the moles you want to convert to (the Al2O3).