we are given with 7 moles of a compound with a molecular formula of al2(so4)3 and is as asked here to determine the number of moles of Al, S and O. In a mole of aluminum sulfate, there are 2 moles of Al, 3 moles of S and 12 moles of O. Hence in total, there are 14 mol Al, 21 mol S, and 84 mol O.
