You can assume pressure around 1 atm.
ok so molar mass gas = 201.79 g/mol
No. But I am not able to reproduce the correct answer either
, and as I calculated it in the Dalmatino bar using a napkin, I have no notes to consult.
Trick is, 10.2 g is not a weight of the added gas - it is the increase of the weight, so you should take buoyancy into account. As the volume is 1.22 L, and the air density is around 1.2 g/L, buoyancy is around 1.22*1.2=1.46 g - so the weight of the gas is around 10.2+1.4=11.6 g. That gives molar mass of around 230.
It should be 15.05-1.22*1.2=13.6 g, I am changing the wording of the original question. Sorry about that.