Final answer:
The mass within a sealed plastic bag would remain the same after a chemical reaction due to the law of conservation of mass, which states that matter is neither created nor destroyed in the reaction.
Step-by-step explanation:
The law of conservation of mass, a fundamental principle in chemistry, dictates that matter is neither created nor destroyed in a chemical reaction. When a chemical reaction occurs within a sealed plastic bag, this principle ensures that the total mass of the system remains constant. Despite the formation of new products as reactants interact, the sealed environment prevents any mass from entering or leaving the bag.
Consequently, the mass within the bag remains unchanged throughout the entire reaction process. This concept underscores the importance of understanding the conservation of mass in closed systems, highlighting the stability and predictability of mass transformations within a confined space, ultimately contributing to the foundational principles of chemical reactions and their quantitative aspects.