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### Course: Health and medicine>Unit 4

Lesson 2: Gas exchange

# 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.

## Want to join the conversation?

• How can we exactly define Henry's Law in short?
• 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.
• 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?
• 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

Sources:
• 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?
• 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!
• 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?
• In my chemistry class, I was also given the formula C=Kh*p
(1 vote)
• 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?

• 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.
• 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?
• i think mole fraction is one of the way in which we can describe the concentration of solution
• 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?
• The concentration would decrease relative to the surface concentration. This is due to diffusion.
• 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?