Here's why:

* The chemical formula SO₃ tells us there is one sulfur atom (S) and three oxygen atoms (O) in each molecule of sulfur trioxide.

* A mole is a unit of measurement for the amount of a substance. One mole of any substance contains 6.022 x 10²³ (Avogadro's number) of that substance's particles.

* Therefore, one mole of sulfur trioxide contains one mole of sulfur atoms.