You're asking for the molar mass of sulfur trioxide (SO₃), not its mass. Mass refers to the amount of matter in a specific sample, while molar mass is a property of the substance itself.

Here's how to calculate the molar mass of sulfur trioxide:

1. Look up the atomic masses:

* Sulfur (S): 32.06 g/mol

* Oxygen (O): 16.00 g/mol

2. Multiply the atomic mass of each element by its subscript in the formula:

* Sulfur: 32.06 g/mol * 1 = 32.06 g/mol

* Oxygen: 16.00 g/mol * 3 = 48.00 g/mol

3. Add the results to find the molar mass of SO₃:

* 32.06 g/mol + 48.00 g/mol = 80.06 g/mol

Therefore, the molar mass of sulfur trioxide is 80.06 grams per mole.