A mixture is the matter which composed of atleast two compounds or solute which comprises to produce either homogenous or hetrogenous mixture. Homogenous mixture is the mixture contains two or any number of compounds in similar ratio whereas hetrogenous mixture is the mixture contains number of compounds but their ratio is not similar.
Sulfur trioxide is a compound not any type of mixture. Sulfur trioxide is composed of sulfur atom along with three oxygen. Its hybridisation is sp^2 and having planar geometry.
Thus, Sulfur trioxide is a compound not any type of mixture
