Aluminium sulfate, Al₂(SO₄)₃, is used to make water clear…

Step 1. Count. $2$ aluminium, $3$ sulfur and $3 \times 4 = 12$ oxygen atoms.

Step 2. Add. $2 \times 26.98 + 3 \times 32.07 + 12 \times 16.00 = 53.96 + 96.21 + 192.00 = 342.17\ \text{u}$.

Step 3. The name. It is an ionic compound of $\text{Al}^{3+}$ and $\text{SO}_4^{2-}$ ions, with no separate molecules, so we say formula mass.