Boyle's Law question
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You should write to NASA to tell them they don't understand it. That's where the graphic comes from.
Boyle's Law
boulderjohn,
I think dumpsterDiver was being facetious. (See his question mark?)
Safe Diving,
rx7diver
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Allow me to become somewhat over-the-top nerdy here 
My point is, "a change in the mass of the gas" usually isn't very relevant, and besides, it's not quite right. What matters is the number of molecules, not the mass. 1 mol (a pretty big, but clearly defined number of molecules) of a gas may have quite different mass depending on the type of gas. 1 mol of He is 4g, 1 mol of N2 is 28g and 1 mol of O2 is 32g. But they all occupy 22.4L at STP (standard temperature and pressure, i.e. 0 degrees C and 1 atm), which you can easily calculate using the ideal gas law if you know that 1 atm is 101325Pa and R is 8.314.
In fact, the wording "a change in the mass of the gas" can be pretty misleading since it's not the mass, but the number of molecules that count. If I have 50g of EAN64 (why EAN64? Because that's half a mol of N2 to one mol of O2, the same ratio we find in nitrous oxide, which I'm going to use as the other part of my example) I have one and a half mol of gas, and it'll occupy 33.6L at STP. 50g of nitrous oxide - nitrogen and oxygen in the same proportion, but bound to each other - is just 1 mol, and it will occupy only 22.4L at STP. Same mass, different number of molecules.
What I guess you meant is that the ideal gas law tells us the amount (or mass) of gas that's required to give the same respiratory volume when pressure increases, i.e. twice as much at 10m (2ATA) as on the surface (1 ATA).
And, incidentally, the ideal gas law allows us to calculate the density of any gas at any pressure and temperature as long as we know the molecular mass of the gas (how many grams one mol weighs). We don't need no steenkin' density tables
I'm pretty certain you got it right before typing it out, but I have an issue with that statement as it's written.
My point is, "a change in the mass of the gas" usually isn't very relevant, and besides, it's not quite right. What matters is the number of molecules, not the mass. 1 mol (a pretty big, but clearly defined number of molecules) of a gas may have quite different mass depending on the type of gas. 1 mol of He is 4g, 1 mol of N2 is 28g and 1 mol of O2 is 32g. But they all occupy 22.4L at STP (standard temperature and pressure, i.e. 0 degrees C and 1 atm), which you can easily calculate using the ideal gas law if you know that 1 atm is 101325Pa and R is 8.314.
In fact, the wording "a change in the mass of the gas" can be pretty misleading since it's not the mass, but the number of molecules that count. If I have 50g of EAN64 (why EAN64? Because that's half a mol of N2 to one mol of O2, the same ratio we find in nitrous oxide, which I'm going to use as the other part of my example) I have one and a half mol of gas, and it'll occupy 33.6L at STP. 50g of nitrous oxide - nitrogen and oxygen in the same proportion, but bound to each other - is just 1 mol, and it will occupy only 22.4L at STP. Same mass, different number of molecules.
What I guess you meant is that the ideal gas law tells us the amount (or mass) of gas that's required to give the same respiratory volume when pressure increases, i.e. twice as much at 10m (2ATA) as on the surface (1 ATA).
And, incidentally, the ideal gas law allows us to calculate the density of any gas at any pressure and temperature as long as we know the molecular mass of the gas (how many grams one mol weighs). We don't need no steenkin' density tables
If a molecule has mass, which it does, then an increase in mass is an increase in the number of molecules. Conversely, an increase or decrease in the number of molecules results in a commensurate increase or decrease in mass. They are completely interchangeable for the discussion we are having, which is about air or NitrOx.
spoolin01
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Storker, I think you're meaning the terms atomic mass or molecular mass as the constants. As NetDoc points out, mass is a proper loose synonym for number of molecules.
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No, when I mean mass I say mass. When I mean mols I say mols. When I mean molecular mass, I say molecular mass. And I sincerely believe that I know what I'm talking about, because this is smack in the middle of my major (chemistry).
Maybe if you're loose enough. I'm not that loose, and that's why I started my post with a nerd alert.
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As shown, joined on June 2006 but never had such a "heated" ? discussion on anything I posted. Kinda got burned out on forums years ago for the same reason - sometimes things get too crazy - so I don't frequent any of them.
You're new to SB …. right?
I can definitely understand this—especially when otherwise smart people, although made aware that they have misunderstood something, continue to argue that they are essentially correct. What's the point in this? My children are thirteen and eleven now. I see a lot of this behavior in them. I caution them that this behavior is counterproductive. It gets in the way. Discussion which teases out exactly where the germ of the misunderstanding lies is one thing. (A good thing, imho.) Argument for the sake of argument is ... childish. IMHO.
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rx7diver
dumpsterDiver
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yeah I was definitely wrong when I said the shape was wrong. I was thinking it to be linear, but that is NOT correct.
---------- Post added October 31st, 2014 at 07:55 PM ----------
Actually, Boyle's law is a simple inverse relationship: as the pressure increases the volume of gas decreases. It's a small part of the ideal gas law which can be used to quantify a change in the mass of the gas.
Perhaps I should not post before I have had coffee in the morning. The shape IS correct. Edit: AND that should have been obvious to me. The graphic is showing how (for example) the air in your lungs expands at a much greater rate, as you approach the lessening pressure near the surface. We (I) should know it doesn't expand linearly with changing depth.
In addition, I mis-read Pete's comment to indicate that Boyle's law is used to calculate the change in the mass of a gas. He actually was referring to the Ideal Gas Law. To say that the later formula is used to calculate the change in mass of a gas would be hard to interpret.
You certainly can use the Gas law to calculate the mass of a gas, if you know the other parameters in the equation, but I'm not so sure that describing it as a way to calculate the change in mass of a gas is correct, since the graphic reminds us that temp and mass are fixed by saying: the temp and mass "are frozen" - not changing.
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