1. Find the molar mass of SO₃:

* Sulfur (S): 32.06 g/mol

* Oxygen (O): 16.00 g/mol (x 3 = 48.00 g/mol)

* Total molar mass of SO₃: 32.06 + 48.00 = 80.06 g/mol

2. Calculate the mass percentage of sulfur:

* (Mass of sulfur / Molar mass of SO₃) x 100%

* (32.06 g/mol / 80.06 g/mol) x 100% = 40.05%

Therefore, the percent sulfur by mass in SO₃ is approximately 40.05%.