[jamesyboy1990]

2KBr + MnO2 + 2H2SO4 (aq) --> MnSO4 + K2SO4 + 2H2O + Br2

in this reaction, bromine is produced. I added all the reactants and let the reaction go on for two hours in a fume hood. However, when i weighed the dish with the remaining products (expecting some mass to be lost due to the bromine vapor escaping), there was actually more mas than before. Why?

[Borek]

Lousy weighting?

The only thing I can think of is that H₂SO₄ is strongly hygroscopic when concentrated.

[Yggdrasil]

How much more mass was there (1% increase, 10% increase, huge increase)?

I would not expect much Br₂ to escape from the reaction.  Br₂ is barely a gas at RT and is quite happy sitting around in aqueous solution.