I don't think you would actually weigh it. You would put it in a container, then measure the pressure and knowing the density of the gas and the volume of the c

I don't think you would actually weigh it. You would put it in a container, then measure the pressure and knowing the density of the gas and the volume of the container you can deduce the mass. If you want to actually weigh using a scale, then put it in a container, measure the total mass accurately and then subtract the mass of the container and the buoyancy due to the atmospheric pressure gradient. The remainder is the mass of the gas.