A Violation of the Second and Tenth Amendments
The label on my bottle of Lysol Power Toilet Bowl Cleaner says “It is a violation of Federal law to use this product in a manner inconsistent with its labeling.” Isn't such a law a violation of the Tenth Amendment? If I brought it across state lines, there might be a case for such a regulation to be constitutional but I didn't.
On the other hand, maybe they didn't want anybody performing the experiments in The Golden Book of Chemistry Experiments. (“Okay kids. Now we'll learn how to make poison gas out of ordinary household cleaning supplies…”) Of course, that would also make this law a violation of the Second Amendment as well.
On the gripping hand, that still wouldn't be a Federal matter (even ignoring that pesky Second Amendment) unless said gas crossed a state line in toxic quantities. It doesn't make sense to ban a potentially-poisonous gas in nontoxic amounts or otherwise it would be illegal to break wind.