# What is stoichiometry?

See this old answer and links. All chemical reactions observe $\text{stoichiometry}$.
The balancing of chemical equations follows conservation of mass. If there are $10 \cdot g$ of products from all sources, AT MOST there can be $10 \cdot g$ of products. Chemical equations balance mass and charge absolutely, and due to (inevitable) losses on handling, yields are therefore less than quantitative.
In the case of multi-step syntheses (and some chemists specialize in the reactions), even a four step sequence with 90% yield at each step (and a synthetic chemist would gladly accept a 90% yield), the overall yield is reduced to 66% automatically. And this reflects mass equivalence: $\text{garbage in equals garbage out}$.