In the experiment of conservation of mass, the volume of the balloon

I think it should be inflated by a balloon, which is affected by buoyancy in the air.

During the combustion of white phosphorus, a lot of heat is released, and the gas in the bottle expands when heated, which makes the balloon expand and the buoyancy increases, so it is unbalanced at this time; After cooling, the volume is restored and the buoyancy is restored to its original size, so it is balanced.

The latter reaction produces hydrogen, and the balloon expands in volume and buoyancy increases, so the balance is unbalanced. However, the total mass of Zn and H2SO4 participating in the reaction must be equal to the total mass of ZnSO4 and H2 generated. But it needs to be measured at the original temperature and pressure in the bottle.