# Henry’s law | Respiratory system physiology | NCLEX-RN | Khan Academy

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Let’s say you’re taking

a look at the interface between a gas– I’m going to

do in yellow– and a liquid down here in blue. And the liquid I’m going

to use is H2O, or water. And you actually

want to kind of keep your eye on exactly what’s

happening right here. So this is your

eyeball, and you’re watching exactly

what’s happening right at that surface layer. In fact, let me write that

down because it ends up being kind of an important idea. You’re just watching the

surface layer of water. And you really want to make sure

that you keep your eye on how the molecules are moving around. So let’s say you’ve got

some molecules in purple, and you’ve got some green

molecules here as well. And four of each, so overall

it’s 50% purple and 50% green. And down below, you’ve

got some water molecules. Let’s draw some oxygens here. And I’m going to draw

some hydrogens as well. So these are little hydrogens

on my water molecules. So these are H2Os,

and all this is happening in a

giant cup of water. So this is a big cup of water. And the purple and

green molecules represent some sort of molecule. Who knows what kind of gas that

is, but some hypothetical gas. And to think

through this, I want to kind of get to the

idea of partial pressure. So we know total pressure

is one atmosphere, or you could write it as

760 millimeters of mercury. But if I’m only interested

in the green molecules, then I would really rephrase

that as partial pressure. And if I wanted

to calculate what that would be, I

could say, I know that there are 4 green

molecules out of a total of 8, and that is 50% green molecules. And I know that the overall

pressure is 760– actually let me leave it in

the same color– 760 millimeters of mercury. And I’ve got 50%, I

said, that are green. So that means that the

green partial pressure is going to be half

of 760, which is 380. So this is the partial pressure

of the green molecules. I figured it out. And I could actually

complicate this a little bit. I could say, well, what

if I got rid of those two and replaced them

with green molecules? So now the gas is

looking different. I’ve got 6 out of 8

molecules that are green. So what is the new partial

pressure looking like? Well, 6 out of 8 means

that the percentage is going to be different. So I’ve got a new

number here and here. So I’d say 75% is

the new number. And I’ve got 75% times 760 is

570 millimeters of mercury. This is my new partial pressure. And the reason I actually

went through that is because I wanted to

show you a way of thinking about partial pressure, which is

that if the number of molecules in a group of molecules–

if the proportion goes up– then really that’s another way

of saying the partial pressure has gone up. And if you have more molecules,

what does that mean exactly? Well, from this

person’s standpoint, this person that’s watching

this surface layer, they’re going to see,

of course, molecules going every which way. Every once in a while,

these green molecules are going to go down

and into the liquid. They’re going to bounce

in different ways, and just by random chance, a

couple of these green molecules might end up down here

in the surface layer. So that’s something

that you would observe. And you’d probably observe

it more often if you actually have more green molecules. In other words, having a

higher partial pressure will cause more of the

molecules to actually switch from the gas part of this

cup into the liquid part of the cup. So I don’t want to

be too redundant, but I want to point out that

as the partial pressure rises, we’re going to have

more molecules, more green molecules,

going into the liquid. So now let me actually

ask you to try to focus on this little green

molecule, this little fella right here, this guy. Now imagine, he’s just entered

our world of H2O’s, and he’s trying to figure

out what to do next. And one thing he might

do is pop right back out. You’d agree that that’s

something he could do, right? If he entered the

liquid phase, he could also just

re-enter the gas phase. He could leave. And a lot of molecules

want to do that. They want to actually

get out of the liquid because the liquid

is a little stifling. It’s kind of crammed

in there, a lot of H2O molecules around in

this case may not like that. So it turns out you

can actually look up, in a table, this value

called K with a little h. And this H with a little

h is just a constant. So this is just a

constant value that’s listed on a table somewhere. And this K sub h

actually is going to take into account

things like which solute are we talking about. When I say solute,

you basically can think of these green molecules. So which is it? Is it a green molecule or

a purple one or a blue one? What exact solute

are we talking about? And what solvent are

we talking about? Are we talking about water? Or is it dish soap or

ethanol or some other liquid that we’re worried

about in this case? And finally, what temperature

are we talking about? Because we know that molecules

are going to want to leave. Especially molecules that

prefer to be in a gas phase, they’re going to want

to leave the liquid, and they’re going to

do it much, much more if the temperature is high. Because when the temperature

is high, remember, the little H2O molecules are

dancing around and shaking around, And that allows

them to free up and leave. So these are three

important issues. What is the solute? What is the solvent? And what is the temperature? And if you know

these three things, you can actually–

like I said, you could look up in a

table what the Kh is. And that tells you a little

bit about that red arrow. What is the likelihood of

leaving the surface layer? So just as before,

where we talked about going into a liquid, this

is now going out of liquid. So Kh, these values that I

said you can find in a table, tell you about the likelihood

of going out of a liquid. And the partial

pressure tells you the likelihood of

going into a liquid. So if you are

looking now– let’s go back to this person that’s

been very patiently observing. If you’re looking at

this surface layer, you can actually do a good job

of checking how many molecules are entering, how many

molecules are exiting, and you can now

calculate a concentration of the molecule in

the surface layer. You could actually say

something like this– pressure, or partial pressure, divided by

K over h equals concentration. So let me write all this out. Concentration is here. And the other two are what we’ve

already been talking about. The p just partial pressure,

and that is right there. And the K with a little

h is the constant, and that is right there. So that’s this guy. So if you just divide the

two, you can figure out the concentration,

and specifically, I mean the concentration of green

molecules in the surface layer. And what does that

really tell you? OK, so now you figure

out the concentration of green molecules

in the surface layer. What the heck does that mean? Well this, my friends,

this formula– actually, I don’t know

if you recognize it, but this is Henry’s law. So a guy named William

Henry– and actually Henry was his last name– came up

with this fantastic formula. And sometimes you

see it rewritten. You might see p equals

concentration times K with the little h. It depends on how you’re

going to present it, but it’s the same formula. And basically what

it says– and it’s a very clever way

of saying it– is that you can take a look at

the molecules that are going into a liquid and

the molecules that are going to want

to leave a liquid. And basically it

gives you a sense for the concentration of

molecules in the surface layer. In fact, another

way of saying is that there’s a relationship

between partial pressure and concentration

within the liquid. So it’s actually a

pretty powerful way of thinking about it. And I hope that by describing

K with a little h in this way you get a more intuitive

feel for what it stands for.

alexandrenrPost authornew videos!!

thank you, rishi!

J Rolland RobisonPost authorGreat video. I'd like to see a few example problems from you!

Sahand HooshmandPost authorveeerry good!

PrisquaredPost authorSo the concentration is the amount of green molecules in the air?

Renato Alessandro BarattoPost authorExcellent explanation. Thanks! Your work is very much appreciated.

CHIPPost authorA very concise explanation there. I think many people fretting over the supposed accumulation of anthropogenic CO2 in the atmosphere could learn a thing or two from Henry’s law. That CO2 is highly soluble and there exists significantly more in water than in the air, and given the CO2 in water and CO2 in air exist in equilibrium and this equilibrium is reached very fast (take a look at a carbonated drink) then the majority of CO2 we are putting in the atmosphere should be absorbed by the oceans.

Yelena12121Post authorGreat video! Made understanding Henry's Law easy. Thanks!

KevinPost authorUmm in my textbook, Henry's law is written as c=kP not c=P/k. I'm not sure which one is correct, you or the textbook. Could you please clear this up?

George BanzonPost authorI have the same problem as kevsac94. In my book the equation is c=kP not c=P/k. Can anybody explain this?

TheUmQualquerPost authorSuperb guys, really good work!

kevin kimPost authoruhhh…. Mr. Khan. if I am not mistaking I believe that the equation for the Henry's law is wrong. it should be 'concentration = P x K'.

Crimson AzraelPost authorvery good lesson man congratulations!

Sarthak JumdePost authorKevin thats just a constant value… u can name it anything u want K' or Kh

Sarthak JumdePost authorIn the textbook which i refer they have used the same formula… so with this one i support Mr. Khan

AjumiPost authorThank you for this video! Haha my english & chemical knowlege is quite poor and i was able to understand both here. (you've done a good job 😉

Anantha KrishnanPost authoryou started from a jar and ended up in henrys law…..;)

thnxxxxxx

Courtney DavisPost authorThank you so much for this video! Really helped to clarify partial pressure and Henry's Law.

IamDivaChefPost authorCan you explain this in relation to diffusion of respiratory gases?

Fa JaePost authorI think the Henrys law is wrong, in my text book it says c = kP

remusomegaPost authorNOTE: Rishi uses the wrong units for K constant. The units are <M/atm>; therefore the expression is conc=K*P

It is important to get this right, because in Fick's law of diffusion the "D" constant is Solubility/Sqrt(mw)

If you learn it like this, it might screw you up. I don't know anyone who uses these units in Rishi's videos.

Santiago Henao SánchezPost author¡Awesome video! Thanks a lot. ¿Does anyone know whats the software he's using as a "writing paper"?

djsunjiPost authorthanks for the very good explenation. just a question, is this the same for for example beer, you get the small bubbles rising up, is this because of the temperature rise?

苏浩锬Post authorWhy there is no H2O molecule in the gas phase? I mean the partial pressure of the green molecule should be 50%times the total pressure minus the water vapor pressure.

crocketmeowPost authorAnother way to express Henry's Law is C = PS –> P = C/S

because KH is inversely proportional to solubility.

Green EryPost authorI believe there is a little problem with the formula of the law. Well, the one I see commonly is C=kh*P. The one you put on the video have a problem at the moment of doing a dimensional analisys. But, checking in other sources it is possible to find the formula in the way presented here.

César Ignacio Brito de DiosPost authorHello to those who think that the equation presented by Mr. Khan is incorrect. There are several ways to represent the Henry Law equation (including P = KH * C). There are also different ways of presenting the dimensions of the KH constant, even it can also be dimensionless.

Respectfully.

Sofia MoralesPost authorExcellent!I'm not clear on the application of the formula of concentration = P / Kh

If the original formula is Concentration = Pressure per Kh

patricia gamblePost authornot really clear

Khaled OsamaPost authorvery bad

Khaled OsamaPost authorwatch it please whttps://www.youtube.com/watch?v=eaqHNuDMfhc

Jane MichellePost authorif partial pressure increases by adding more molecules of that gas, then…..I don't understand why people say there is the same amount of O2 in the air at high altitude as at sea level?? If the percent of O2 is the same…… Do people mean the percent of O2 in relationship to all other elements comprising atmosphere??? so their ratios are the same but there truly are less O2 molecules in the same amount of space because of pressure?? there is less O2 in a gallon of air at 30k feet than a gallon of air at sea level. AND gases also move along a concentration gradient to equilibrium (between alveoli and cappilaries?

Maria MartinezPost authorcan you provide an actual respiratory example perhaps knocking some one's hypoxic drive?

Naresh PachauriPost authorThe formula is wrong

CaughtlighghtPost authorI like how he makes it sound like all people sit around observing the surface of waters for molecular change

D herreraPost authorI love it!, thanks u so much!!