Explain how the law of conservation of mass is the basis for all stoichiometry?
The Law of Conservation of Mass states that mass can neither be created nor destroyed, it can only be converted from one form to another.
Therefore, whatever you start a reaction with must be present in some form at the end of the reaction. SInce Stoichiometry is the theoretical comparison of the matter and therefore mass of material before a reaction with the matter or mass after a reaction, the mathematical comparisons can be carried out knowing that nothing disappears.
When a log burn is a fireplace and days later you sweep up the ashes to discard them, the mass of the ashes in no way compares with the mass of the original log. However, if we were to take in to account all of the byproducts of the burning log, the steam, the smoke, the resins etc… the mass of all of that matter would be in ratio to the original mass of the log.
Using stoichiometry and comparing a complete balanced chemical equation all mass and matter must be and would be accounted for.
I hope this was helpful.