Question

how many moles of oxygen atoms are present in 5 moles of Mg3(PO4)2

Answer 1

How many moles of oxygen atoms are present in 5 moles of Mg3(PO4)2

All you have to do is to create a ratio between the molecule and the oxygen atom.

5 moles of Mg3(PO4)2 (4x2 moles O/1 mole Mg3(PO4)2) = 40 moles of oxygen