The balloon gets larger and the buoyant force on the balloon is different than it was when it was empty. All expandable containers have this problem. Constant volume containers like glass flasks, don't have this problem, but already have air in them. Problem is how to get the air out. In the laboratory, you will measure the mass of a gas which occupies a certain volume by measuring the difference in the mass of a solid which gives off a gas on heating, and you will measure the mass of a volatile liquid which fills the flask as a gas at a higher temperature.
Pressure is defined as the force exerted per unit area of surface.
Pounds - Force sq.in. Chemists have not wholeheartedly adopted this unit. In the lab, it is a glass tube filled with Hg, then turned upside down in a dish of Hg. Hg flows out until the weight of the mercury in the tube is supported by the atmospheric pressure acting on the surface of the mercury in the dish. A gas occupies all the volume enclosing it.