What happens to mass during chemical and physical changes?
Not a lot......................
In every chemical and (non-nuclear) physical process EVER observed, mass is CONSERVED. That is, if you start with
Organic chemists, who regularly perform multi-step natural product syntheses, are well aware of these problems. They might start out with kilos of starting material, but in a multistep synthesis, it would not take too many steps, even with +80% yields, before they are dealing with milligram quantities.