Question

How does the law of conservation of matter apply to chemical equations?

Answer 1

It applies absolutely: `"Garbage in must equal garbage out!"`

Explanation:

If I start with `10*g` of reactant from all sources, at most I can get `10*g` of product. In practice, I am not even going to get that. Why? Because for every reactant particle, with a given mass, there must be a corresponding product particle with equivalent mass, though losses invariably occur on handling. Mass is thus conserved in every chemical reaction, though losses (big and small) invariably occur on handling.

See this old answer.. The idea of mass equivalence is a very important one, and as chemists we should always strive to optimize our yields. Of course, we are never going to succeed.