# What is the number of oxygen atoms in Al_2(SO_4)_3?

There are 12 oxygen atoms in one formula unit of $\text{Al(SO"_4)_3}$.
The subscript for the sulfate ion $\left({\text{SO"_4}}^{2 -}\right)$ is $3$. The subscript for the oxygen atom is $4$. Multiply them together to determine the number of oxygen atoms in one formula unit of aluminum sulfate. $\left(3 \times 4 = 12\right)$