Can a balance accurately measure the mass of a gas in a balloon?

[platina]

Is it possible to use a balance to measure the mass of a gas?

Let's say that the experiment I'm going to describe is possible and takes place in a vacuum so as to avoid any buoyant force from the surrounding atmosphere, and the balloon to be used does not allow any effusion of the gas from within. The balance to be used has infinite precision.

If you take the mass of a balloon (not inflated, not tied off), you are measuring the mass of the balloon (the latex or whatever).

If you inflate the balloon with pure carbon dioxide gas and tie the balloon off and place it on the balance what are you taking the mass of? Is it possible that the balance is measuring the mass of every single CO2 molecule in the balloon?

Here is my reasoning...

I can imagine if it were possible to condense every last CO2 molecule in the balloon into dry ice while it sat on the balance, the balance would report the mass of the latex and every CO2 molecule.

If then, the dry ice was able to sublime would the reading on the balance slowly decrease as CO2 molecules entered the gas phase?

I'm struggling to see how the mass of those gas particles not in contact with anything (in transit from one end of the balloon to the other) could effect the reading of the balance.

Thanks for your time...this idea was prompted by the conservation of mass experiment involving baking soda, a balloon, vinegar, a bottle, and a balance.

 

[NobodySpecial]

I'm struggling to see how the mass of those gas particles not in contact with anything (in transit from one end of the balloon to the other) could effect the reading of the balance.

Their weight presses down on the gas molecule just below them, which presses down on the one below that - until you get to the bottom which presses on the balance.

It's the same picture of how do the molecules in the middle of the balloon not touching the walls contribute to the pressure

 

[sophiecentaur]

Take a large (5l) round bottomed flask and evacuate it. Weigh it. Then fill it with the gas at a known pressure. Weigh it again. That will give you the mass of gas inside.

I have done this many times using air and the difference in the empty and full weights is easily enough to give a good idea of the mass of air in the flask. Kids (some of them, anyway) are gobsmacked when you show them that air actually does weigh something. A wardrobe of air weighs a couple of kg.

That, of course, is using a rigid container. If you use dry ice and let it evaporate into a baloon, then it will displace more and more air as its volume increases so it will experience more and more upthrust. But the balloon will weigh more when full of CO2 than when empty or full of air. Assume a 'rigid' / floppy balloon (or a poly bag), rather than a rubber one because the pressure caused by the resilience of the rubber as it inflates needs to be taken into consideration in the more complex case.

"I'm struggling to see how the mass of those gas particles not in contact with anything (in transit from one end of the balloon to the other) could effect the reading of the balance.
"
Good question. The molecules are constantly on the move and bumping into each other. The pressure is transmitted through the gas at the speed of sound in the gas - so every molecule in the balloon is contributing to the pressure on the sides. The sum total of forces on the balloon is a downward force on the balance, comprising the weight minus the upthrust due to the air molecules hitting it from the outside. All side to side forces will cancel out - both inside and outside, so the balloon will not move sideways but there is a net downward force (its weight) due to collisions inside and out with molecules.