What happens if the mass does Increase in a Unsealed Chemical Vessel?

-If the Mass increases, the most likely reason is that one of the Reactants is a Gas, that's in the Air. The products also probably Aqueous, Solid or Liquid -Before the Reaction take place, the Gas is floating in the Air. Even though its there, its not being noted in the Reaction Vessel, so you don't know the Mass -When the Gas reacts, it becomes in the Product, therefore being in the Reaction Vessel. This increases the Total Mass of the Reaction Vessel -An example can be if a Metal reacts with Oxygen, in a Unsealed Container. The mass in the Container Increases, because the Metal Oxide is the equal mass of the Metal and the Oxygen in the air