First we break down Al2(SO4)3 and look at SO4 only. In this compound, there are already 4 moles of O. Then you see that there are 3 moles of (SO4), meaning there are 3*4 moles of O = 12 moles in the entire compound. Then you look at the question, where it's asking for 2.9 moles of the entire Al2(SO4)3 compound, so you multiply 2.9*12 = 34.8 total moles of O.
This is true, vapor is just evaporated water. Like in machines, you have to fill it up with water so your machine can produce the water vapor. Hope this helps!!
