Basic Gas Cylinder Question

[Maverikk]

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Ok, so I've been reading recently and can't figure this out:
Book's say to use Boyles Law (PiVi=PcVc) to figure out the volume in an oxygen cylinder. Pi=Initial Pressure, Vi=Initial Volume, Pc=Current/Gauge Pressure, Vc=Current Volume.
This does not work, what you use is Vi/Pi=Vc/Pc
What's going on here?

 

[deleted171991]

The volume of gas while in the cylinder stays the same. Hence the pressure decreases with the quantity of gas remaining in the cylinder. It's measured in L at 1 atm for convenience, so that you know how much you still have available for patient use, but that is not the volume of the gas while in the cylinder (which is always the volume of the cylinder).

The remaining O2 could be measured in any unit that is proportional to the number of molecules remaining in the cylinder, which are the ones driving the pressure. The more interaction these molecules have with the cylinder wall (more molecules/higher temperature/lower volume), the higher the pressure.

That was the phenomenon, here is the formula to "prove" it: The ideal gas law says that PV=nRT. Since here V and T are constant, P/n is also a constant. Hence the pressure decreases with the moles of gas remaining in the cylinder.

Boyle's law does not apply here. Boyle's law applies when you squeeze a balloon.

Homework: Knowing that an E-cylinder contains 660 L of O2 at about 2200 psi (150 atm), at room temperature, calculate the internal volume of the cylinder. (I promise you'll be surprised.)