Antoine lavoisier, the french scientist credited with first stating the law of conservation of matter, heated a mixture of tin and air in a sealed flask to produce tin oxide. did the mass of the sealed flask and contents decrease, increase, or remain the same after the heating?

Remain the same

Explanation:

The law of mass conservation states that matter cannot be created nor destroyed but can be converted from one form to another.

Essentially, what Lavoisier was trying to proof is that by heating the mixture, after all the change the mass still remains. That was why he used a sealed flask. If the flask was not sealed, it probably would have been that some of the mass will escape as vapor to the atmosphere which might be difficult to account for