Talk:Ideal gas law

Name: Universal gas equation
I changed the name of the article from Universal gas equation (UGE) to Ideal gas law (IGL) for the following reasons: CO GDEN  01:05, Nov 19, 2004 (UTC).
 * IGL is in much wider use than UGE. For proof of this, a search of "universal gas equation" on http://scholar.google.com yielded one hit (and that was in a farming education manual), while a search of "ideal gas law" yielded 3,110, and it's used in every thermodynamics textbook I've seen.
 * The IGL isn't in fact "universal". There isn't anything universal about it, because it's a simplification. In fact, no gas actually follows it, just an imaginary ideal gas.

Derived n
Is n=(PV)/(RT)? Someone should put up the derived formulas for n,P,V,R, and T. That would help a lot.

--

Why? The equation is ridiculously simple as it is - finding any of the five just involves some very simple algebra. It would just clutter the article to state them explicitly. --193.11.221.16 09:51, 5 Jan 2005 (UTC)

the pressure needs to be in atmospheres, not pascals.


 * I agree, after all, atm is SI -- Comtraya 01:06, 17 February 2007 (UTC)

No. The SI unit for pressure is the pascal, not the atm. The formula nR=PV/T shows that actually there are two SI units of measurement for amount of substance: nR measured in J/K, and n measured in mol. The gas constant R is the conversion factor, telling how many J/K make one mol. Bo Jacoby 00:12, 20 February 2007 (UTC).

Noting that PV=nRT is an approximation
I think it would be very helpful to change the equality to an approximate equality. Denoting that the law only works for ideal gases is important - because some people somehow never fully understand what "ideal" means. Being still a student - I find the use of approximations taught to kids as being truths is uterly ridiculous. People should learn what is and isn't an approximation - and then if they care, they can look up the more accurate equations. Fresheneesz 08:00, 21 November 2005 (UTC)

I respectfully disagree. The gas constant R is known with many significant digits. If you change some of these digits the formula is still approximately true, but not exactly true. So PV=nRT is more than an approximation. It is exact in the limit. Bo Jacoby 10:06, 21 November 2005 (UTC)

Ideal Gas is hypothetical and does not exist in any form in reality and therefore the Ideal gas law is an approximation. "The approximation breaks down at high pressures and low temperatures, where the intermolecular forces play a greater role in determining the properties of the gas." taken from [Ideal gas]. It is important to note that the ideal gas law is accurate but not precise. Physics is a description/approximation of reality, it is not a prescribed rule. Furthermore most laws and theories are unable to account for a number of physical phenomenons. --ANONYMOUS COWARD0xC0DE 05:12, 29 November 2006 (UTC)
 * Seems I didn't completely read the first statement now that I look at it again. I have to agree with Bo Jacoby, although the equations approximate reality the equations are themselves exact.  That said it is still important to note that ideal gases are approximations of reality.  So I will compromise by adding the statement "a hypothetical gas" (same wording from Ideal_gas) after the first use of the phrase "ideal gas".  --ANONYMOUS COWARD0xC0DE 06:03, 23 December 2006 (UTC)

Edit by 134.82.125.70
Please log in to identify yourself. I do not agree that the preferred unit of pressure is the atm, because the SI says pascal. I don't think that the values of the gas constant should be quoted here because it has an article of its own. The inaccuracy of the gas law should not obscure the fact that the gas law determines the gas constant with many significant digits. It is more than just an approximation: It is asymptotically exact. The statement: At high temperatures, there are high energy collisions between particles, making intermolecular forces seem negligible, is not true. Even a single molecule constitutes an ideal gas. Bo Jacoby 08:56, 5 December 2005 (UTC)

Rewrite
I went ahead and rewrote everything in LaTeX. I removed the references to $$ k_B \ $$ because that information is more relevent in a discussion of $$ R \ $$. Also, I don't know who wrote the passage about the ideal gas law not covering condensation and thus being less accurate than the Van der Waals equation. The correct reasoning is on the page now.

Let me know if there's something missing or incorrect. Cheers! Isopropyl 05:51, 2 March 2006 (UTC)

Added alternate forms
I added the alternate forms section, showing the common derivations using molar mass, or specific gas constants. Also, I made note that the temperature has to be in K or R... That's something that always nips me when working with this. KMossey 03:28, 8 June 2006 (UTC)


 * the other parameters need to be in terms of R too, if you are a fan of R. maybe a comment to that effect needs to be added? Xcomradex 03:31, 25 July 2006 (UTC)

Inconsistency in validity range of IGL
The paragraph states that the equation holds for monoatomic gases. Two sentences later it states that the temperature needs to be high so intermolecular attraction is neglectable. I suppose single atoms could be called molecules in some cases but I think attractive forces are not present between single atoms in conditions related to the IGL, as they are due to polarization of "polyatomic" molecules. I am a bit confused because I have always been thought about the law applied to monoatomic gases, but lately I have red about the role of intermolecular attraction (Vacuum Science and Engineering, C. M. Van Atta, McGraw-Hill Book Company (1965), page 17). I think somebody should clarify this as the validity range of the IGL seems to be an important topic. Preymond (talk) 02:54, 24 March 2008 (UTC)

Hard-sphere gas is not an ideal gas
The hard-sphere gas is, in fact, a much-analyzed non-ideal gas. It probably deserves an article of its own. It should *not* be confused with the ideal gas.


 * ✅, as apparently someone created hard spheres some time ago. David Hollman (Talk) 15:28, 9 September 2010 (UTC)

Volume in PV=nRT
Volume should be specific volume ?? check the units —Preceding unsigned comment added by 132.244.246.25 (talk) 15:03, 2 April 2008 (UTC)

Um hey, I’m new at this and I probably don't know much but here goes nothing.

This article states that volume in PV=nRT is measured in cubic meters (1000's of Liters) but in school I was taught that you express it in just L or cubic dm. Further more I was taught the same value for R. I’m not sure but this would affect calculations wouldn't it? Zippypic 03:55, 4 May 2007 (UTC)

Typically, the equation will be in Liters and either atms or bars (10^5 Pa) just because they are covenient. I can see using bars over atmospheres because bars represent an actual SI unit, but reducing everything to the base units is just annoying. - anon


 * P in troy pound square knot per firkin,
 * V in US dry pecks,
 * n in half dozens and
 * T in Planck temperatures
 * ... if you like. As long as you've got the right units for R, it'll work fine. Jɪmp 08:44, 23 January 2008 (UTC)


 * Okay, not so helpful, right.
 * If you're using R = 8.314 472 J·mol−1·K−1 or 8.314 472 m3·Pa·K−1·mol−1 then your volume will be in cubic metres (or kilolitres), pressure in pascals, temperature in kelvin, n in moles.
 * If you're using R = 0.082 057 84 L·atm·K−1·mol−1 then your volume will be in litres & pressure in atmospheres (T & n as above).
 * If you're using R = 62.3637 L·mmHg·K−1·mol−1 it's litres & millimetres of mercury (T & n same again).
 * If you're using R = 10.7316 ft3·psi·°R−1·lb-mol−1, it's cubic feet, pounds per square inch, degrees Rankine and pound-moles.
 * Happy calculating. Jɪmp 08:53, 23 January 2008 (UTC)

Pressure units on the Ideal Gas Law
I've been browsing the web, and while doing some calculations using the law, I noticed the the article about the law seems to say that the unit for pressure is pascals, but every other web page, book, and so on and so on says that the unit is kPa, not Pa. I'm thinking that someone may want to look into this. Capcom1116 (talk) 03:18, 20 August 2010 (UTC)
 * It's OK. The k in kPa is just a prefix meaning "1,000". For example, atmospheric pressure can be written as 101.325 kilopascals (kPa), 101,325 pascals (Pa), 10,132,500 centipascals (cPa), or whatever; all are equally correct.  What the article is saying is that the basic SI unit for pressure is the pascal, and not the meter or the gram or something.  As for what prefix is used in front of that, well that's up to the individual! (Personally I think petapascals is the most fun to say!) Riick (talk) 15:25, 20 August 2010 (UTC)

n = number of moles?
I think I can see where Adamtester is coming from about p not being the "number of pascals" but rather the amount of pressure (non-unit specific), and then n not being the "number of moles" but rather the amount of substance. But I think he's wrong, and the point is a little bit subtle. The difference between the ideal gas constant and Boltzmann's constant specifically is the fact that the ideal gas constant is expressed in moles - it's Boltzmann's constant multiplied by Avogadro's number. This is touched on in the introduction, I've seen it expressed in various SI and English units, but it's always per mole. I recommend changing the definition of n back to "number of moles". On a related topic, on the gas constant page, they say that R "is equivalent to the Boltzmann constant, but expressed in units of energy (i.e. the pressure-volume product) per kelvin per mole (rather than energy per kelvin per particle)." I'd be more comfortable with "units of energy per temperature increment per mole". I think I'll go suggest that .... Khakiandmauve (talk) 16:16, 8 March 2011 (UTC)
 * $$\frac{PV}{T}=nR$$ is the amount of gas measured in the SI unit joule per kelvin, J/K. R is the conversion factor between J/K and mol. There is only historic reason why the mole was defined in the first place, and it is incomprehensible why it was made a SI unit. Bo Jacoby (talk) 15:36, 1 March 2012 (UTC)
 * You mean a fundamental unit? Mole being a SI unit is not to be objected, but being fundamental is. Mole is a unit like radian, a dimensionless unit. Also there is little justification for candela being a fundamental unit. The SI documents state about these two units that they of practical importance, not very fundamental.--5.15.59.0 (talk) 10:24, 20 January 2016 (UTC)

Symbol consistency
It bugs me a little that in the Alternative Forms > Engineering section, the notation gets screwed up. Would it be reasonable to write the equation of state using Rspecific (the same as is used at specific gas constant), and then note that engineers often use the symbol Rbar to refer to the ideal gas constant, and the symbol R to refer to the specific gas constant? That way, at least all the equations on this page use consistent symbols. Khakiandmauve (talk) 16:33, 8 March 2011 (UTC)
 * Went ahead and changed it, and while I was at it, combined the Engineering section with Molar form. Although I think the title is wrong (and was wrong previously) - to me, molar form seems to imply that R is in units of inverse moles, which is the normal form.  I suggest retitling it "Specific gas constant form".  I'll think about that for a while. Khakiandmauve (talk) 21:33, 8 March 2011 (UTC)

Microscopic derivation
(copied here from my talk page for others to chime in... Vsmith (talk) 15:49, 3 February 2014 (UTC))

I've noticed your edits to ideal gas law and I consider it is important to specify explicitly that the derivation of ideal gas law is microscopic and to distinguish between macroscopic laws and theoretical derivations involving microscopic considerations.

Also the macroscopic status of the ideal gas law is the same as that of, for example, the law of gravitation and can be taken as a primitive statement/law needing no macroscopic derivation. --188.26.22.131 (talk) 15:35, 3 February 2014 (UTC)


 * You are welcome to specify explicitly with a reference noting such microscopic considerations. However, the derivation is mathematical and of itself doesn't depend on a microscope. Vsmith (talk) 15:43, 3 February 2014 (UTC)

continued reply:
 * A reference is not (really) needed to improve the clarity/context of the derivation as it need no derivation on macroscopic scale, it can be taken as a basic law. The microscope is also not needed to make the theoretical distinction between macroscopic scale and microscopic scale. (Another detail about derivation seems to be necessary to specify in the context: Derivation makes use of logical consequences, thus is first logical and secondarily mathematical).--188.26.22.131 (talk) 15:59, 3 February 2014 (UTC)

original research removed from article
Removed the following until a reliable secondary source can be found. Nearwater (talk) 19:48, 11 April 2014 (UTC)

Universal gas constant was discovered and first introduced into the ideal gas law instead of a large number of specific gas constants by Dmitri Mendeleev in 1874.

N in the statistical mechanics should be number of molecules not number density
I'm pretty sure that the N in PV = NkT is just the number of molecules and not the number density, am i mistaken? http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/idegas.html says the same, and that is also how i was originally taught. also, is there a reason for the inconsistency in boltzmann constant sometime having the subscript b and sometimes not? (in the same section), or is the k a different constant (although it still seems to have the same value as boltzmanns constant) for the formulae towards the end of the statistical mechanics section.

31.54.40.234 (talk) 21:44, 14 May 2015 (UTC)

Status of this law
Is this law a law of thermodynamics or not? There seems to be a confusion between a physical law and a principle.--5.15.59.0 (talk) 10:43, 20 January 2016 (UTC)

The Ideal Gas Law is an example of an Equation_of_state, not a "law" of thermodynamics. Willbmoore (talk) 16:22, 8 January 2021 (UTC)

There I see a revolutionizing gas equation
One researcher is coming to new equation https://www.academia.edu/30504157/A_New_Equation_in_Thermodynamics, my team has crosschecked the word, it seems ground breaking,

Please any administrator contact the researcher to allow to be featured on wiki, he was quite ignorant otherwise it will be copyright infringement. This researcher is damn correct at his findings, bloody great head.

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internal energy sectoin is incorrect.
internal energy of a gas = 3/2 nRT only when the gas is monatomic. Rotation of the molecules (for more complex molecules) results in factors like 5/2 or 7/2 instead. also, its poorly written. this is an extremely big mistake to make! Please fix this. — Preceding unsigned comment added by 2602:306:BCC4:2230:4F8:C168:6DB:7B48 (talk) 22:00, 16 May 2017 (UTC)
 * Some more explanation on that score might be appropriate, but isn’t an ideal gas effectively monoatomic by definition? It’s been a long time since I studied physical chemistry, but IIRC the particles of an ideal gas are supposed to be like geometric points, which would preclude their possessing angular momentum, or any form of kinetic energy other than translational.—Odysseus 1 4 7  9  00:47, 17 May 2017 (UTC)

The majority of books define "ideal gas" as p=R*T*rho and internal energy e some function of T. (Examples: Liepmann and Roshko, "Elements of Gas Dynamics", p. 8+9, or Courant and Friedrichs, "Supersonic Flow and Shock Waves", p.6). Some books are more restrictive, defining "ideal" as e proportional to T, which would be called "polytropic" elsewhere. Even that would permit f/2 nRT with f other than 3. (Not a few texts have sloppy definitions of "ideal" and "real", so only sampling enough standard books helps.) So I suggest to clarify that 3/2 is only for "monatomic" gases. — Preceding unsigned comment added by 140.109.104.238 (talk) 09:39, 30 March 2019 (UTC)

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History missing
A historical account of the ideal gas law is missing.MaoGo (talk) 12:34, 15 September 2018 (UTC)


 * Yes, I came to this article because I wanted to read about the history of this law. At first there were the separate equations like Boyle's Law and Charles' Law, but who was it that first combined them, and when? When did it get named "Ideal Gas Law"? 2001:8B0:DFDC:12BC:759:79A4:937E:9CC8 (talk) 12:17, 7 January 2023 (UTC)

Improper derivation
The first derivation via the combination of the simpler gas laws is not valid. While it is a nice way to motivate the idea for a general chemistry level course, it should be noted that the proportionality relationships used are only valid if the other variables are fixed. This means that they cannot be algebraically combined in the way claimed. The derivation from the Boltzmann distribution is much more proper, though it is questionable whether the “rms” is actually a part of the speed. 2603:7080:E935:FBA2:B435:B1E0:D0B6:204D (talk) 11:49, 9 May 2023 (UTC)

Ideal gas law, general gas equation, ideal gas equation
Hi, is "ideal gas equation", as used in section "Applications to thermodynamic processes", another synonym? 2A0A:A541:7977:0:588D:A945:81A3:84AF (talk) 09:19, 31 May 2023 (UTC)

Ideal gas law, general gas equation, ideal gas equation
Hi, is "ideal gas equation", as used in subsection "Applications to thermodynamic processes", another synonym? 2A0A:A541:7977:0:588D:A945:81A3:84AF (talk) 09:19, 31 May 2023 (UTC)