The of a gas depends on its mass and its volum (of course), which means that it depends on pressure and temperature.

When you are at STP, 1 mole of any ideal gas occupies exactly ##”22.4 L”##. SInce no other information is given, you can safely assume that you are dealing with 1 mole of sulfur trioxide (##SO_3##).

Now, if you have 1 mole, we’ve established that you have a volume of ##”22.4 L”## at STP. All you need now is its mass – for this use ##SO_3##’s molar mass – ##”80.1 g/mol”##.

This tells you that 1 mole of sulfur trioxide weighs ##”80.1 g”##.

Therefore, the of sulfur trioxide at STP is

##rho = m/V = (“80.1 g”)/(“22.4 L”) = “3.58 g/L”##

This is a rather lengthy tutorial, but very good, it does not have the sulfur dioxide per se: http://www.eiu.edu/eiuchem/genchem/tutorial6.pdf