sulfur trioxide is SO3 atomic mass: S = 32.1 g/mol O =16.0 g/mol molar mass of SO3 = 32.1 + 16.0x3 = 80.1 g/mol moles of SO3 = 2.15 mol mass of SO3 = moles of SO3 x molar mass of SO3 = 2.15 x 80.1 = 172 g Solution sulfur trioxide is SO3 atomic mass: S = 32.1 g/mol O =16.0 g/mol molar mass of SO3 = 32.1 + 16.0x3 = 80.1 g/mol moles of SO3 = 2.15 mol mass of SO3 = moles of SO3 x molar mass of SO3 = 2.15 x 80.1 = 172 g.