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# O2 and CO2 solubility

## 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 decided 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 no 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 are some molecules of oxygen here and you know 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 you know 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 the concentration at that level is zero point two seven 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 give it in units that that I like so I'm going to write the units of the K H down here I can say well 769 liters times atmospheres over moles that's something that I've just calculated I have 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 kind of challenge myself and I 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 but imagine I can actually do that you know 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 okay 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 molecule is going to settle in here and of course the atmosphere is not going to run out of carbon oxide molecules they're just going to keep replacing them but they keep settling into this top layer the surface layer of my water so it's actually looking already really different than what was happening on the other side right we only add a little bit of oxygen but now I've got tons of carbon dioxide and I don't want to make it uneven you know I mentioned before we have nitrogen so let me still draw a bunch of nitrogen that will outnumber the carbon dioxide dramatically right because we have you know here about let's say 79% nitrogen and we only a 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 point 2 4 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 right on the oxygen side the number was actually pretty small not very impressive and yet on the carbon dioxide side much much higher now that's kind of funny it might strike you is kind of a funny thing because look these partial pressures are basically the same right 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 four 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 time on the spheres / moles so it's a much lower number and I don't want you to you know get so distracted by this bit this is kind of irrelevant to what we're talking about is 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 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 h2 co3 well think about that if it's binding to the water then it's not going to want to leave it's pretty comfortable being in the water right and so the moment the 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 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 more soluble than oxygen I'll put that in parentheses then oxygen and 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 Celsius but what about 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 ash they shouldn't be writing water instead of water it's blood which is slightly different than water right 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 turns out that carbon dioxide is about 22 times 22 times more soluble 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 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