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As the gas is released from the can, the gauge pressure and mass of gas within the can will decrease.
Gas molecules create pressure by colliding with the surface within the can. Releasing the gas from the can decreases the number of gas molecules that are able to collide and create pressure. The number of moles of gas within the can can be calculated using the atmospheric pressure, room temperature, and the volume of the butane can. The molecular weight of an unknown gas can be estimated by the ratio of the mass of the gas in the can to the number of moles calculated.