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You have an evacuated container of fixed volume and
known mass and introduce a known mass of a gas sample.
Measuring the pressure at constant temperature over time,
you are surprised to see it slowly dropping. You measure the
mass of the gas-filled container and find that the mass is
what it should be—gas plus container—and the mass does
not change over time, so you do not have a leak. Suggest an
explanation for your observations.