Answer : The mass of sulfur trioxide is, 488.8 grams.
Explanation : Given,
Moles of sulfur trioxide = 6.11 mole
Molar mass of sulfur trioxide = 80 g/mole
Formula used :

Now put all the given values in this formula, we get the mass of sulfur trioxide.

Therefore, the mass of sulfur trioxide is, 488.8 grams.