Real gases and the van der Waals equation | Physical Processes | MCAT | Khan Academy

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[osama]

For anyone having trouble understanding what he said about pressure or for a clearer understanding of the pressure concept for real gasses read below. Like if this helped you.




The pressure we observe in real gasses during an experiment is not the same pressure we need to use for the equation PV=NRT. this is because molecules are attracted to each other which slows down their velocity as they hit the wall of the container with less force decreasing the observed pressure(force/Area). So for a real gas the P observed does not equal P ideal used in the formula. P observed is less than P ideal. To correct for this we add the term a(n/v)^2. you might ask why do we even need to add a term? the reason is pressure is supposed to tell us something about the energy of the gas molecules and energy is directly related to the temperature of the gas. Keeping the volume constant and the number of moles constant, pressure is directly proportional to temperature. The observed or experimental pressure is not related directly to the kinetic energy of the gas because not all molecules in a gas slow down in a real gas. if you take a molecule in the center of the gas they will not slow down because the attractive force from all other particles will cancel out. only when a gas molecule is about to hit the container or is further away from the center will there be a net attractive force acting on it that will slow it down. therefore our experimentally measured observed pressure is a lot lower than what we should expect given the ideal gas Law PV=NRT. PV will not equal NRT, in fact, PV will be less than NRT so PV/NRT will be less than 1.

[GuitarMartin]

This is a great video! Every important aspect of the equation is covered in a simple and understandable way, and that's what I love about you guys, you are so magnificently pedagogic in the ways you teach:)

[Wellknow]

It's van der ( vaals) as a pronounce w in German is pronounced as v

[josephclark]

That was a beyond excellent explanation!!! Sincerely thank you!!!

[jasminegallagher]

Kahn Academy you have saved my life so many times

[shardulshinde]

nice video very easy to understand . Thanks sir

[abdulaiyahuzanantomah]

Thanks for this great video.Explained in a very simplified manner that ease understanding.

[lakshmireddy]

I didn’t not understand that if the force is increased the particles concentration will increase @5:20 and you did square the formula n2/v2

[Mizelleful]

You mentioned that the ideal volume is obtained by subtracting from the actual volume of the container nb. Wouldn't this result in a corrected volume instead of an ideal volume, which is associated with the presence of an ideal gas that takes up no volume? I would expect the ideal volume = volume of container, and the corrected volume for a real gas is volume of container- nb. Just a point I think should be clarified.

[justinjambles]

Possible mistake found?
Imploding systems: ie, the particles are attracted to each other, would mean that the correction needs to be subracted form the ideal pressure? And vice versa for repellent systems? If this statement is correct, than I believe you misspoke in the video because you said the exact opposite. 

[satvinderpalsingh]

But pv=nrt is an empirical formula where p, v were the pressure and volume observed by the scientists. So they are the real volume and real pressure. So Pv=nrt should be a correct relation for real gases if observed experimentally. Plz help

[raihanhasbilla]

Could you explain the work formula of this gas?

[rishitsherekar]

In ideal gas, we had assumed that there's no effect of gravity on the molecules.
What about that?
Can there be a correction for that?

[gowtham]

Was little bit fast..
Real tym examples wud ve been intuitive....
Great job...thank you soo much

[remusomega]

You made a mistake
The volume of the container is the 'ideal' volume. That container volume minus the volume of the gas (nb) will give you the real volume.

[remusomega]

And you didn't even explain what "v" is given the (n/v). Is that the ideal volume or the container's volume?

You did a real bad job on this one.