# partial pressure

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– I am not wrapping my head around partial pressure – according to wikipedia:

“the notional [pressure](https://en.wikipedia.org/wiki/Pressure) of that constituent gas if it alone occupied the entire [volume](https://en.wikipedia.org/wiki/Volume) of the original mixture at the same [temperature](https://en.wikipedia.org/wiki/Temperature)”

does it mean the pressure of the existing quantity of constituent gas if all other gases were removed from the given volume? Not the pressure if you were to fill that space to capacity with the gas?

In: Physics

Say you have a big tub filled with ping pong balls. Half of them are red, a quarter are blue, and a quarter are green. The ping-pong balls are pressing against each other and the sides of the tub. How hard they press against the sides is the pressure.

Now, say you took out all the red and blue balls. How hard are the green balls pressing against the sides of the tub now? The tub takes up the same amount of space as it always did, and there’s the same amount of green balls, but there’s fewer balls overall, so that force is smaller. That’s the partial pressure.

As for “filling the space to capacity”… with gas that doesn’t make any sense. Gasses always expand to fill their container (ignoring gravity or other external forces). You may be thinking as if the gas were an incompressible liquid, which can completely “fill up”. Adding more gas will just increase the pressure, and the “max capacity” of the container is just when the pressure inside gets so high the container isn’t strong enough to hold it anymore and ruptures or explodes.

It’s how much a given gas contributes to the overall pressure.

If you have, say, a container at 10 psig with a mix of N2, O2, and CO2, then if the partial pressure of N2 were, say, 3 psig, then that would mean 3 of the 10 total psig of pressure is due to the nitrogen. Or put another way, if you removed all the O2 and CO2 and just left the same amount of N2 in the same sized container at the same temperature, it pressure of that container would only be 3 psig.

> Not the pressure if you were to fill that space to capacity with the gas?

Gases spread out to fill the shape of their containers. You can’t *not* fill that space to capacity with gas, which is why it’s more helpful to measure gas in moles or pressure rather than volume.

Yes to your first answer, no to the second. Imagine a 100 foot tall
open-topped tank filled 2/3 with rocks and 1/3 with water (by volume).

The water pressure at the bottom of the tank, under 100′ of water and rocks,
would be 3 atmospheres. Now remove the rocks.
The water pressure at the bottom, under 33′ of only water would be 1 atmosphere.
The water is the “constituent gas” and the rocks are the “other gases”.

Same pattern as a sealed tank of air, if a relative of Maxwell’s daemon
could remove all gasses except oxygen from the tank, the partial
pressure of the remaining O2 would be around 20% of the original pressure.