The density 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
Now, if you have 1 mole, we've established that you have a volume of
This tells you that 1 mole of sulfur trioxide weighs
Therefore, the density of sulfur trioxide at STP is
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