[stremba]

Chemistry courses are focused on the old law of conservation of mass and that is drummed into the students heads so often even the teachers start to believe it.

That's probably because, as an approximation, the law of conservation of mass holds pretty well in chemical reactions. You wouldn't be able to actually measure the mass loss in any reaction. This law is a simpler one to teach chemistry students than the law which actually holds. The law of conservation of mass (as well as the law of conservation of energy) has been superseeded by the law of conservation of mass-energy, but there's really no reason to introduce that distinction into an introductory chemistry course.

[WildTurkey]

but there's really no reason to introduce that distinction into an introductory chemistry course.

But it is actually taught that that E=MC²applies only to nuclear reactions and not to chemical reactions.