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Let's say you had 3 identical air tight bottles, each has a one way valve. What happens if you?

Let's say you had 3 identical air tight bottles, each bottle has an air tight one way valve. If bottle # 1 has 3/4 of it's air vacuumed out, bottle 2 leave the valve open so air pressure is the same inside and out, bottle 3 you pump in some helium. Do all 3 bottles weigh the same when put on a scale?

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You didn't specify the amount of helium within the third bottle, as for the other two, they would be identical in weight, they take up the same amount of space, but if you really want to get technical and measure by the atoms within, lightest would be helium, assuming it has at least 3/4 of it helium and 1/4 air, next lightest is 3/4 empty 1/4 oxygen, and heaviest bottle is the one that is unchanged, simply filled with oxygen.

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Yeah I didn't know how to specify the amount of helium :-)
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It doesn't matter how much helium you add. The key is that you are adding more gas, so the weight will increase. The weight would only go down if you took out some air and replaced it with He, but your question says that no air was removed from bottle #3.
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No they would not equally be balanced but great question!!

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