How many moles of O are present in 2.9 mol of Al2(SO4)3 ?

34.8 moles of O

Explanation:

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.