Final answer:
A fuel cell obeys the law of conservation of mass by ensuring the total mass of reactants is equal to the total mass of products, exemplified when hydrogen and oxygen react to form water with no mass gain or loss.
Step-by-step explanation:
A fuel cell is a device that converts chemical potential energy (fuel) into electrical energy through an electrochemical reaction that typically involves hydrogen and oxygen. The question pertains to how a fuel cell adheres to the law of conservation of mass. This fundamental principle in chemistry expresses that, during any chemical reaction, the total mass of the reactants must be equal to the total mass of the products. Therefore, when a fuel cell operates, the hydrogen fuel reacts with oxygen to produce water, electricity, and heat, and the collective mass of these products is equal to the mass of the reactants used.
To further illustrate, if one were to balance the chemical equation of the reaction occurring inside a fuel cell, for example, hydrogen combining with oxygen to form water (2H2 + O2 -> 2H2O), it would clearly show that the atomic mass of the hydrogen and oxygen reactants equals the atomic mass of the water product. The conservation of mass is exemplified in this process, as there is no mass gain or loss during the reaction, despite the transformation of substances.