What means equilibrium of a balance?
The masses on the two sides are the same.
On the left side we see always the same mass. The flask situated on the right side is always the same. What does that mean for the contents of this flask ?
The masse of these contents remains constant.
Deduce Lavoisier's law!
Lavoisier's law: The mass of the reagents of a chemical reaction equals the mass of the products.