Question

Which Property of Air is demonstrated by the following explanation: An empty balloon weighs less than a blown up balloon.

Answer 1

Explanation:

First the air in the balloon is not massless. It weighs something.

Now if you mean you want to count the buoyancy (i.e. a helium balloon has negative weight), then a filled balloon will still weigh more than an empty balloon because the elasticity causes the air inside the balloon to be under more pressure than the outside air and thus have greater density.

I actually did this as an experiment when I was in grade school. The filled balloon actually tips the balance because it does weigh more even with buoyancy taken into account.