# How many oxygen atoms are in Al_2(SO_4)_3?

There are 12 oxygen atoms in $A {l}_{2} {\left(S {O}_{4}\right)}_{3}$.
In $A {l}_{2} {\left(S {O}_{4}\right)}_{3}$, the subscript of three next to the $S {O}_{4}$ polyatomic ion refers to the number of that same ion is in this molecule. Since there are four atoms of oxygen in each $S {O}_{4}$, indicated by the subscript of four next to the oxygen, and there are three $S {O}_{4}$ ions, $4 \cdot 3 = 12$ oxygen atoms present in $A {l}_{2} {\left(S {O}_{4}\right)}_{3}$.