74 votes

Why would breathing pure oxygen be a bad idea?

The other answers here, describing oxygen toxicity are telling what can go wrong if you have too much oxygen, but they are not describing two important concepts that should appear with their ...
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62 votes
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Why can we still breathe in valleys?

It does. You would find the average percentage of the atmosphere that is argon is very slightly higher at the floor of valleys. However, bear in mind first of all it wouldn't be anywhere near a ...
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48 votes
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What volume does one mole of an ideal gas occupy?

The common saying is a hold over from when STP was defined to be $\pu{273.15 K}$ and $\pu{1 atm}$. However, IUPAC changed the definition in 1982 so that $\pu{1 atm}$ became $\pu{1 bar}$. I think the ...
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32 votes
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How could the ideal gas law be discovered from experiments on real gases?

The ideal gas law is a very good approximation of how gases behave most of the time There is no logical flaw in the laws. Most gases most of the time behave in a way that is close to the ideal gas ...
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29 votes

Why would breathing pure oxygen be a bad idea?

Our body is used to the environment around us. Once you change part of the environment, you have to be ready for the consequences. Inhaling pure oxygen is the cause for what is known as oxygen ...
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  • 10.1k
24 votes

Why is the van der Waals coefficient b equal to four times the volume of the particle?

While most everything the previous answer states is correct, I would point out that taking four times the volume of a single particle has nothing to do with experiment and arises mathematically. In ...
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20 votes
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PV=nRT approximation on other planets?

The differences in acceleration due to gravity is not the main factor in comparing how accurate the approximation is for each planet. The main factor is the mass of gas each planet's atmosphere ...
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19 votes

How could the ideal gas law be discovered from experiments on real gases?

You must consider this: The question whether a physical system follows a particular law is not a "yes or no" question. There is always an error when you compare what you measure with what the law ...
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  • 291
18 votes

What volume does one mole of an ideal gas occupy?

A big point of confusion is that it is still taught (at least in the mid-2000's) that STP is defined with respect to $\pu{273 K}$ and $\pu{1 atm}$ of pressure, or $\pu{1.01325 bar}$ of pressure, even ...
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  • 1,716
17 votes

Relation between constant-pressure and constant-volume heat capacities: Cp - Cv = nR

Preliminaries Consider $U = U(V,T, p)$. However, assuming that it is possible to write an equation of state of the form $p = f(V,T)$, I don't have to explicitly address the $p$ dependence of $U$, ...
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17 votes
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Why do helium balloons expand in volume as they go higher?

I didn't know that balloons expanded during the fly because of thermodynamics, and I didn't know how high they can fly, but a rapid search tells that a partially unfilled regular balloon can fly until ...
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  • 3,467
16 votes
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Relation between constant-pressure and constant-volume heat capacities: Cp - Cv = nR

The heat capacities are defined as $$C_p = \left(\frac{\partial H}{\partial T}\right)_{\!p} \qquad \qquad C_V = \left(\frac{\partial U}{\partial T}\right)_{\!V} \tag{1}$$ and since $H = U + pV$, we ...
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16 votes
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Why don't heavy and light gases separate in the atmosphere?

That's because of two reasons. One is entropy, the ultimate force of chaos and disorder. Sure, gases would like to be arranged according to their density, but even above that, they would like to be ...
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  • 29.6k
16 votes

Why does gas not liquify at a temperature above the critical temperature no matter how much pressure is applied on it? Why?

The critical point is a point of convergence of all state properties of the respective liquid and gas. It can be considered as the degeneration point, where there is no difference between gas and ...
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  • 27.3k
15 votes

Why would breathing pure oxygen be a bad idea?

As a certified SCUBA diver, I learned that breathing pressurized pure oxygen leads to oxygen toxicity, which can be fatal. However, I'm not anywhere near an expert on the mechanism of oxygen toxicity, ...
15 votes
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Is oxygen above the critical point always supercritical fluid? Would it still appear to roughly follow the ideal gas law?

Yes. Any fluid with a temperature is above critical temperature and the pressure above the critical pressure is by defintion a supercritical fluid. Don't be mislead by all the claims that ...
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  • 20.2k
14 votes
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When I heat up a balloon, does the air inside increase in pressure as well as volume?

If the balloon is closed, then yes, both volume and pressure will increase when the gas inside is heated. Let's look at two simpler cases first. If the gas were completely free to expand against ...
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14 votes
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According to KMT, is the velocity of an ideal gas always sqrt(3RT/M)?

$E=\frac 12mv^2 \implies v=\sqrt{\frac{2E}{m}} $ is valid for translational kinetic energy and the speed of the centre of mass. Vibrational or rotational energy does not count. An object may vibrate ...
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14 votes

Why does gas not liquify at a temperature above the critical temperature no matter how much pressure is applied on it? Why?

It really does liquefy. But it does not do so in exactly the same way as you see below the critical temperature and pressure. As an example, suppose you heat steam to 400°C and then compress it, ...
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  • 38.3k
14 votes
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Can a vacuum on a closed system really “pull” things out of it?

Gasses don't get "pulled", they get "pushed". When you empty a neon tube, you attach a device with almost no air in it to mouth of the tube. The air that is in the tube then pushes ...
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13 votes
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Do 1 mole of both dioxygen gas and its atoms occupy 22.4 litres at STP?

Does this mean that both 1 mole of $\ce O$ would occupy $22.4~\mathrm L$ (or if this doesn't usually occur in nature, say 1 mole of $\ce{He}$ or another monoatomic gas) 1 mole of $\ce{O2}$ would ...
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  • 16.8k
13 votes
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Reason for negative Joule Thomson coefficient of Helium and Hydrogen at NTP conditions

Note: You can skip section I, and go straight to section II and/or the end of section III (specifically the conclusions subsection), if you are already familiar with the basic mathematical machinery/...
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13 votes
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Why is dp/dv zero for a real gas in critical conditions?

This is merely a shard of a fact which does not make much sense in and by itself. After all, in systems with gas/liquid equilibrium there is nothing really special about $\left(\dfrac{\partial\...
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12 votes

When I heat up a balloon, does the air inside increase in pressure as well as volume?

You may recall the ideal gas law: $$PV = nRT.$$ Here, $P$ is pressure, $V$ is volume, $n$ is the amount of gas present (in moles), $R$ is the ideal gas constant, and $T$ is temperature. In an ...
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11 votes
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Why does CO2 lowers the pH of water below 7?

Carbon Dioxide (CO2) readily dissolve in water and form Carbonic Acid (i.e H2CO3 (aq) ) This is the formation of bonds. Then Carbonic Acid (i.e H2CO3 (aq) ) dissociate in water as follows. So ...
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10 votes
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Determine the molecular formula of the compound using percentage composition

You're actually on the right track. Looking at the percent composition, you've correctly identified that the ratio of $\ce{C}$ to $\ce{F}$ atoms is 1:1, however, you cannot assume that the formula is ...
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10 votes
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Vapor Pressure Paradox

Although "paradox" is not quite the right term, what you have discussed is actually a simple, yet interesting and important phenomenon. Given the ideal situation as you have presented, your ...
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10 votes
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Can the van der Waals coefficients be negative in the van der Waals equation for real gases?

This question requires a simplistic notion of real gas behavior. The van der Waals equation was based on the notion that "real" gas particles occupy some volume, and have an attraction to each other....
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9 votes
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Rearrangement of a gas law equation

If one rearranges the ideal gas law equation, you can obtain the following (assuming $n$ and $T$ are non-zero): $$\frac{PV}{nT} = R$$ $R$ is a constant, and there are in fact infinitely many ...
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  • 11.6k
9 votes
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What's the difference between perfect and ideal gas?

An ideal gas is the same as a perfect gas. Just different naming. The usual name for such gases (for which is assumed that the particles that make up the gas have no interaction with each other) is ...
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