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Henry's law

Explore the relationship between partial pressure of a gas and the concentration of the gas molecule within a liquid. Rishi is a pediatric infectious disease physician and works at Khan Academy. These videos do not provide medical advice and are for informational purposes only. The videos are not intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of a qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read or seen in any Khan Academy video. Created by Rishi Desai.

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  • leaf green style avatar for user Rupam
    How can we exactly define Henry's Law in short?
    (21 votes)
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    • leaf blue style avatar for user Peterson
      Henry's Law has to do with the principles involving dissolved gas and pressure. It states that the amount of gas dissolved under equilibrium in a certain volume of liquid is in direct proportion to the pressure of the gas that makes contact with the liquid's surface.
      (39 votes)
  • blobby green style avatar for user edc1g13
    I'm not sure if I understand this video has gas concentration = Kpa / solubility coefficient whereas my lecturer has the formula has [Gas]= Kpa x solubility coefficient. Do you know which is correct?
    (22 votes)
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    • hopper cool style avatar for user SofiyaMarkova
      The confusion may be that the same law can be expressed in different ways based on how you look at it.

      K = Henry’s constant (found on charts)
      P = partial pressure of the gas (solute)

      There is:
      S = KP

      S = concentration of gas (solute) in the SOLUTION (solvent) [ex. water]
      - How likely be in solution

      There is: [video]
      C = P/K

      C = concentration of gas in the AIR
      - How likely be in air

      The relationship of being in the solution or in the air is inversely proportional because:

      If measure that # of particles in air go up = # of particles in solution go down [the particles can only go 2 ways so if more in one, less in other]

      If measure # of particles in air go down = # of particles in solution go up

      (17 votes)
  • female robot grace style avatar for user Hypernova
    This isn't totally relevant to the video, but this question popped into my head when Rishi said that molecules moved from the air to the cup. Here's my question: how do molecules move? What are they propelled by? And for that matter, what are the electrons "orbiting" an atom being propelled by?
    (5 votes)
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    • blobby green style avatar for user Chris Boris Alexander Wied Tøstesen
      It's a question about physics and to a lesser degree chemistry.

      The answer can go in a lot of directions, and by some rationale can be said to be unanswered. I always want to remind myself of inertia though.

      We tend to take something like friction and gravity for granted, so we percieve motion to be this temporal thing that is always moving towards it's own end.

      Of course when we think about it we know that nothing has any change in velocity unless it is acted upon. We actually don't really know why that is, it just seems to be one of the attributes of the universe as we experience it. Apart from that all mass has gravitational pull and particles of the size of atoms and molecules care strongly about forces like electromagnetic force.

      I don't know how far you are in physics but to give a very rudimentary explanation you could take something like why do the atoms in a gas keep moving around? If you imagine a large game of pool, and then imagine the table to be much much more perfect so that almost no friction at all is there, can you see how the balls would simply keep moving? For energy to dissapear there has to be something to "take over" that energy. Like another ball or the tablecloth on which the ball is moving or the air around it.
      When the cloth takes it over, it now has that energy in one form or another. Temperature, Static electricity, all ways in which we experience the energy.

      There is some energy in these things, and it all came from somewhere. It won't neccesarily keep on moving. Vapor could turn into water and then ice by having other molecules and atoms near it, to which the kinetic energy could be effectively transfered. Apart from our basic idea of atoms and molecules, something like photons could also interact with these through forces that we are gradually understanding - like forexample the neat microwave ovens which very effectively stimulate "movement" of water molecules. In other words raising their temperature and thus being an excellent tool when we specifically want to put heat into food, which by being a biological compound tends to have a load of water in it.

      When talking about what propels an electron i have to be careful not to seem like i know more than i do. Through quantum physics we obviously know that it doesn't even neccesarily move in the way we normally think things do. From a to b, in a trackable fashion. Rather it has a probability equation for it's placement that we can look at. This is the danger of talking about electrons as actually orbiting around something, and why we call their probable placements "orbitals" instead of just orbits, like the movements of planets.

      Hope that made a little sense at least!
      (17 votes)
  • leaf green style avatar for user Chris D
    In my textbook, and on Wikipedia, Henry's law is written like Cx=ax*Px --> ax (or Kh)=Cx/Px. Why could it also be written like this?
    (11 votes)
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  • leaf red style avatar for user Jennifer Ness
    What creates a surface layer at a molecular level? Do all solvents have a surface layer? Does the surface layer of water have any special properties?

    Edit: Found something! https://www.khanacademy.org/test-prep/mcat/physical-processes/fluids-in-motion/v/surface-tension-and-adhesion
    (3 votes)
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    • leafers ultimate style avatar for user Sneha Rajendran
      I wouldn't really think of the "surface layer" as a tangible thing with defined dimensions. What he is referring to when he says surface layer is just the relatively topmost part of the solvent in which the most gas molecules from the atmosphere can dissolve. At the molecular level, the surface layer can be distinguished from the rest of the container due to the fact that it contains the most number of particles from the atmosphere.
      (4 votes)
  • blobby green style avatar for user Alvin Lin
    I'm confused. in my textbook it says Henry's law is : P=KH*X, where X is the mole fraction. so the mole fraction and the concentration are the same thing?
    (4 votes)
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  • purple pi teal style avatar for user Olivia
    How steeply does concentration fall off as you move away from the surface layers? If I go ten centimeters down into that water, how can I figure out the concentration of green molecules there?
    (3 votes)
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  • blobby green style avatar for user gvaldez0290
    When I look at p my physics book and look up Henry's law online I get a different equation. C=KP not C=P/K. Which is the correct equation?
    (2 votes)
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  • aqualine seed style avatar for user Ben Hall
    How are we able to find the value for the constant (Kh) just by itself?
    (2 votes)
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  • leafers seedling style avatar for user Sukriti
    What is the difference between Henry's Law and Raoult's Law?
    In my textbook, Henry's law is stated as " the partial pressure of the gas in vapour phase is proportional to the mole fraction of the gas in the solution", while Raoult's Law is stated as " for any solution the partial vapour pressure of each volatile component in the solution is directly proportional to its mole fraction"
    (1 vote)
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    • blobby green style avatar for user chebbard1
      My understanding is that Raoult's law is talking about the pressure an individual, trapped component in a single solution/liquid/gas exerts (on walls) while Henry's is talking about how the pressure of an individual component--with a choice to move into one of 2 phases/compartments--might cause the component to move from one phase to another. For Raoult's, I think about air trapped in a glass bottle. The total pressure felt by the bottle is P = RT*n/V, where n = all moles of gas (O2, N2, CO2...). If you wanted to know how much of that total pressure was caused just by O2, you'd have to find the moles of O2 or subtract the N2, CO2 etc from the total. Henry's seems more like phase partitioning. Now you have a bottle with half water and half gas. If you increase the amount of O2 above the water, you're going to end up pushing some O2 into the water, and the amount that gets pushed into water vs the amount that stays in the gas just depends on the equilibrium constant for that particular setup.
      (2 votes)

Video transcript

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.