Question on lifting power of gases

[Yapper]

take a balloon filled with helium if you where to make the balloon rigid, without changing its weight, then suck half of the helium out of it. would the balloon, with the same volume but different density, have more lifting power?

 

[Doc Al]

What produces the "lifting power" is the buoyant force due to the air pressure acting on the balloon. That buoyant force depends on the volume of air displaced, not on what's inside the balloon. But the load that the balloon can lift is reduced by the weight of the balloon material plus whatever is inside the balloon. So by removing some helium, the balloon can lift an additional load equal to the weight of that removed helium. (Even better would be to remove all the helium! Realize that this is a thought experiment only--lightweight, rigid balloons that can withstand atmospheric pressure while containing a vacuum are hard to come by. )