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


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.

38 comments

  1. A 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.

  2. Umm 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?

  3. uhhh…. 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'.

  4. Thank 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 😉

  5. NOTE: 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.

  6. thanks 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?

  7. Why 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.

  8. I 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.

  9. Hello 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.

  10. Excellent!I'm not clear on the application of the formula of concentration = P / Kh
      If the original formula is Concentration = Pressure per Kh

  11. if 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?

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