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O2 and CO2 solubility

Get an intuition for why carbon dioxide is so much more soluble than oxygen when it goes into water. 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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  • aqualine sapling style avatar for user Simy
    I know you are meant to divide partial pressure by concentration to get Kh but how exactly did you get 769 at and 29 at ? I seem to be getting a different answer
    (12 votes)
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    • blobby green style avatar for user Pedro Marvão
      I think the main problem has to do with the units. Concentration is given in mmol/l (that is mol x 10-3) and Kh units are in mol. So if we take that into account you should get 777 for Oxygen and 29 for carbon dioxide. The disparity between 769 and 777 can be accounted for if we assume that the concentration value that was used had some extra digits (e.g. 0.2731 mmol/L). So the values for the equation for Oxygen would be 0.21 atm / 0.0002731 mol/L.
      (11 votes)
  • blobby green style avatar for user vanlaikhup
    How about the concentration of nitrogen in our blood?
    (7 votes)
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    • leafers tree style avatar for user Roger Gerard
      Nitrogen (N2) is an extremely stable molecule due to it's triple bond. Due to it's stability the molecules on Nitrogen that diffuse through the blood do not undergo many, if any, chemical reactions in the blood.

      To answer your questions, the amount of Nitrogen we inspire and expire is approximately 79%, due to the reason state above.
      (16 votes)
  • mr pants teal style avatar for user Patrick M

    Doesn't there need to be carbonic anhydrase to catalyze the reaction of CO2 and H2O into H2CO3
    (3 votes)
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  • purple pi teal style avatar for user Olivia
    The difference in the ratios of CO2 solubility to O2 solubility between room temperature and body temperature- is that more due to CO2 becoming less soluble at warmer temps, or O2 becoming more soluble?
    And if you have hypothermia, is the increase in CO2 solubility or decrease in O2 solubility pronounced enough to lead to acidosis or hypoxia, respectively? Does that contribute at all to death from hypothermia?
    (5 votes)
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    • starky ultimate style avatar for user ♪♫  Viola  ♫♪
      In regards to your first question, gasses will generally become less soluble at higher temperatures because they will have more kinetic energy which allows them to bounce around in a gaseous form.
      I would assume that the solubility of both CO2 and O2 would decrease at higher temps. but less so CO2 because much of the CO2 molecules will have already converted into bicarb and H+.
      (3 votes)
  • blobby green style avatar for user Ariel Lecky
    at Henry's law is written as Pressure over concentration= kH. However i learned that concentration over pressure =kH which formula is correct?
    (5 votes)
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    • orange juice squid orange style avatar for user Brandon Quinn
      The way you learned it and the way it is presented are really two sides of the same coin. It all comes down to how you want to express the answer. You can rearrange the formula in many different ways depending on the situation and how you want to express the units.

      In your learning of Henry's law the units were expressed in mols(gas) / L(solution)*atm whereas in the video the units are expressed in L(solution)*atm / mol(gas).

      This is why it is very important to express your units and check when you use a constant that you are in the correct units to use it. I have made this mistake many times in undergrad.

      source: https://chemengineering.wikispaces.com/Henry%27s+Law
      (2 votes)
  • winston default style avatar for user David
    At , I start to get really lost, Could someone please explain to me how Rishi came up with the number 769 and the term after it? Thanks!
    (4 votes)
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  • blobby green style avatar for user smariah252
    What is concentration and nitrogen in our blood
    (2 votes)
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  • male robot donald style avatar for user Kevin
    If Carbon Dioxide is more soluble in blood then Oxygen, why does Oxygen go into the blood vessels, and Carbon Dioxide leaves during respiration.
    (2 votes)
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  • blobby green style avatar for user Rebecca Barrett
    Isn't most of the O2 carried by our blood carried bound to hemoglobin? So why do we care about the solubility of O2 in the blood? Isn't that not really realavent, because O2 dissolving directly in the blood contributes so little?
    (1 vote)
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    • leaf blue style avatar for user dysmnemonic
      Yes and no. We care the most about haemoglobin for determining the oxygen carrying capacity of the blood, but the Hb mostly acts as a buffer for dissolved oxygen. When dissolved oxygen is taken out of the blood for cells to use, it's replaced by O2 dissolving back into the blood from Hb. O2 dissolving into blood and fluids is also important for the uptake of oxygen in the lungs, as the oxygen has to get to the Hb inside the erythrocytes to be carried around.
      (2 votes)
  • blobby green style avatar for user raghusanjay1
    how many grams of co2 gas is dissolved in a 1lt of carbonated water if the manufacturer uses a pressure of 2.4 atmosphere in the bottling process at 25degrees celsius given kh value of co2 is 29.76 atm/mol/l at 25degrees celsius
    (1 vote)
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Video transcript

Let me do a little experiment. Let's say I have oxygen here, and we know that oxygen is about 21% of the atmosphere. And I decide to take a cup, let's say a cup like this-- simple cup of water. And I leave it out on the counter. And it's about room temperature, about 25 degrees Celsius here. And I want to know, how much oxygen is really going to enter that cup at that surface layer? So let's say I want to measure the concentration of oxygen in that surface layer of water. Well, you know, I say 21% so, of course, there's some molecules of oxygen here. And it's only 21%, it's not like it's the majority. So I've got to draw some other molecules. This could be nitrogen or some other molecule, let's say. But I'm focused on the blue dots, because the blue dots are the oxygen dots. And so over time I let this kind of sit out. And maybe I come back and check, and a little bit of oxygen has entered my surface layer of water. In fact, if I measured it, I could say, well, the concentration, C, at that level is 0.27 millimoles per liter. And this number is literally just something that I would have to measure, right? I would actually measure the concentration there, and that's the measure of oxygen. So I've learned about Henry's law, and I can think well, you know, I know the partial pressure now. And I can rearrange the formula so that it looks something like this. I can say, well Henry's law basically is like that. So if I know the pressure and I know the concentration, I should be able to figure out the constant for myself. I can figure it out and kind of give it in units that I like. So I'm going to write the units of the KH down here. I can say, well, 769 liters times atmospheres over moles. And that's something that I've just calculated. I've just taken two numbers and I've divided them by each other. So this is my calculation for oxygen. And so far, so good, right? But now I decide to challenge myself and say, let's do this again. But instead of with oxygen, I'm going to create an environment that's 21% carbon dioxide, which is way more carbon dioxide than we actually have. But imagine I could actually do that. I actually find a way to crank up the carbon dioxide, and I do the exact same thing. I take a cup of water and I keep it out at room temperature, 25 degrees Celsius. And I say, OK, let's see how much carbon dioxide goes into my cup. I've got my carbon dioxide out here. And over time more and more molecules kind of settle in here. And, of, course the atmosphere is not going to run out of carbon dioxide molecules. They're just going to keep replacing them. But they keep settling into this top layer, this surface layer, of my water. So it's actually looking already really different than what was happening on the other side. We only had a little bit of oxygen but now I've got tons of carbon dioxide. And I don't want to make it uneven. I mentioned before, we have nitrogen-- so let me still draw a bunch of nitrogen-- that will outnumber the carbon dioxide dramatically. Because we have here about, let's say, 79% nitrogen and we only had 21% carbon dioxide. So it'll look something like that. But there's lots and lots of carbon dioxide there. In fact, if I was to calculate the concentration on this side, the concentration would be pretty high. It would be 7.24 millimoles per liter. And Again, these numbers, I'm assuming that I'm doing the experiment. This is the number I would find if I actually did the experiment. So it's a much bigger number than I had over here. On the oxygen side, the number was actually pretty small, not very impressive. And yet on the carbon oxide side, much, much higher. Now that's kind of funny. It might strike you as kind of a funny thing. Because look, these partial pressures are basically the same. I mean, not even basically, they're exactly the same. There's no difference in the partial pressure. And yet the concentrations are different. So if you keep the P the same, the only way to make for different concentrations is if you have a different constant. So let me actually move on and figure out what the constant is. So what do you think the constant on this side would be, higher or lower? Let's see if we can figure it out together. The K sub H on this side is going to be lower. It's going to be lower. It's 29 liters times atmosphere divided by moles. So it's a much lower number. And I don't want you to get so distracted by this bit. This is kind of irrelevant to what we're talking about. It's just the units, and we can change the units to whatever we want. But it's this part-- it's the fact that the number itself on the carbon dioxide side is lower. Now let's think back to this idea of Henry's law. Henry's law told us that the partial pressure, this number, tells you about what's going to be going into the water, and that the K sub H tells you about what's going out of the water. And so if what's going in on both sides is equivalent, then really the difference is going to be what's the leaving. And on this side, on the first side of our experiment, we had lots of oxygens leaving this water. They didn't like being in water. They were leaving readily. And so you didn't see that, but they were actually constantly leaving. And on the carbon dioxide side, you had maybe a little bit of leaving, but not very much. The carbon dioxide was actually very comfortable with the water. In fact, to see that as a chemical formula, you might recall this. Remember, there's this formula where CO2 binds with water and it forms H2CO3. Well, think about that. If it's a binding to the water then it's not going to want to leave. It's pretty comfortable being in the water. And so the moment that carbon dioxide goes into water, it does something like this. It binds to the water. It turns into bicarbonate and protons. And so it's a very comfortable being in water, and that's why it's not leaving. In fact, I can take this one step further and even compare the two. I could say, well, 769 divided by 29 equals about 26. So that's another way of saying that carbon dioxide is 26 times more soluble than oxygen. I'll put that in parentheses-- than oxygen. And I should make sure I make it very clear. This is at 25 degrees Celsius, and this is in water. Now, you might say, well, that's fine for 25 degrees Celsius. But what about body temperature? What's happening in our actual body? What's happening in our lungs? So in our lungs, we have 37 degrees Celsius. And instead of water-- actually, I shouldn't be writing water-- instead of water, it's blood, which is slightly different than water. The consistency is different. And so these K sub H values are actually temperature dependent. And they're going to change as you increase the temperature. So at this new temperature, it turns out that carbon dioxide is about 22 times more soluble than oxygen. So it's still pretty impressive. Sometimes you might even see 24 times, depending on what numbers you read. But this is an impressive difference. And actually, what I wanted to get to is the fact that it goes back to the idea of what's going in and what's coming out. And the net difference is why you end up with a huge difference in concentrations between carbon dioxide and oxygen.