The gas that is not acidic is CO, or carbon monoxide. Acidity is determined by a substance's ability to donate hydrogen ions (protons), and thus, increase the hydrogen ion concentration in a solution. In general, gases that contain oxygen atoms, such as SO3 and NO2, are more likely to be acidic because oxygen has a greater tendency to form bonds with hydrogen atoms. CO, on the other hand, does not contain oxygen and is not acidic.