When the density formula at STP is:
density = molar mass / molar volume
So we can get the molar mass of SO3 from the periodic table = 32 + (16x3) = 80g
and at STP each 1 mole of gas occupy 22.4 L so we have the molar volume at STP = 22.4.
So by substitution in the density formula:
∴ density of SO3 = 80 g / 22.4 L = 3.571 g /L
