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

It applies absolutely: $\text{Garbage in must equal garbage out!}$
If I start with $10 \cdot g$ of reactant from all sources, at most I can get $10 \cdot g$ 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.