How does stoichiometry relate to the law of conservation?
Excellent question regarding stoichiometry and the law of conservation of mass.
According to the Law of Conservation of Mass, in "ordinary" chemical reactions, mass is neither created nor destroyed.
Stoichiometry deals with the chemical quantities of a BALANCED chemical equations.
The following is an example of an unbalanced equation and we can see why it does NOT follow the law of conservation of mass.
From the equation above, it appears that you begin with 2 hydrogen atoms and form 2 hydrogen atoms. That's just fine. BUT, you cannot begin with 1 oxygen atom and end up with 2 oxygen atoms on the product. This is a violation of the Law of Conservation of Mass.
Below, you will find the balanced equation for this reaction which DOES follow this law.
In the above example, you begin with 4 hydrogen atoms and produce four hydrogen atoms; you start with 2 oxygen atoms and end with 2 oxygen atoms.
The coefficients needed to balance this chemical equation tell you the ratios that the reactants and products exist in; stoichiometry deals with the quantities of balanced equations.
Hope this helps!