Sometimes, when given this type of question, it is easiest to assume that you have 1 liter volume. Then we can use the ideal gas law, PV = nRT and solve for moles and then convert to grams.
P = pressure = 90.0 kPa x 1 atm/101.325 kPa = 0.888 atm (converted to atm so can use R = 0.0821)
V = volume = 1 L
n = moles = ?
R = gas constant = 0.0821 Latm/Kmol
T = temp in K = 45 + 273 = 318K
Solving for n (moles) we have...
n = PV/RT = (0.888)(1)/(0.0821)(318)
n = 0.034 moles SO2
molar mass SO2 = 32 + 32 = 64 g/mol
Grams SO2 present = 0.034 mol x 64 g/mol = 2.18 g
Since we have a volume of 1 liter, the density will be 2.18 g/L