On what law is stoichiometry based, and how do the calculations support this law?
See this old answer and links.
Every chemical reaction that HAS EVER BEEN PERFORMED obeys conservation of mass. You start with
In practice, you are not even going to get that. Losses invariably occur on handling. And note that this is an experimental result, that is rationalized by our particle view of chemical reactivity. We don't have to rely on calculations; we do have to rely on experiment.