The given chemical reactions outline the conversion of sulfur to sulfur dioxide (SO2), then to sulfur trioxide (SO3), followed by the formation of sulfuric acid (H2SO4) and the reaction with sodium hydroxide (NaOH) to form sodium sulfate (NaSO4). Finally, barium chloride (BaCl2) is added to the solution to precipitate out barium sulfate (BaSO4), leaving sulfur trioxide (SO3) in the solution.