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

-If the mass Decreases ,it might be because some/all the Reactants are Solids, Liquids or Aqueous, and that one of the Products is a Gas -Before the Reaction, your Products are Contained in the Reaction Vessel -If the Vessel is Open to Escape, the Gas can follow suit when its been Formed. Since its not in the Reaction Vessel, its Mass can't be taken in -So the Total Mass of the Reaction Vessel decreases -An example can be a Metal Carbonate decomposing to make a Metal Oxide, and Carbon Dioxide -If its not Sealed, the Carbon Dioxide can go away, making the Mass look Less