Answer:
In the case of a balloon, the helium inside the balloon weighs less than the same volume of air the balloon displaces. ... So a helium balloon has two opposing forces acting on it: gravity pulling it down, and buoyancy pushing it up. Buoyancy wins, so you can't really weigh a balloon, even though it does have weight.
Knowing the volume of air displaced by the helium balloon, we can calculate the maximum mass the balloon can lift. ... Helium, which has a mass of 4.00 g/mol, has a density of 0.164 g/l. Thus, a one-liter balloon of helium can lift a mass of (1.18-0.164) = 1.02 g.