Can an open vessel be filled with gas to 40% of its capacity?

Gas fills the entire volume provided to it, that is, in the case of an open vessel, it will spread throughout the room. In addition, the capacity of even a closed vessel when it is filled with gas is a relative value and depends on the pressure at which the gas is pumped (at one pressure, the capacity will be one, at another, another).

Based on the above, the answer is impossible.

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