Understanding the Different Forms of the Ideal Gas Law and Their Applications

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Discussion Overview

The discussion revolves around the different forms of the Ideal Gas Law and their applications, particularly focusing on the various constants (R values) associated with each form. Participants explore the implications of using specific volume and the notation inconsistencies encountered in different educational resources.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant seeks a method to determine which form of the Ideal Gas Law to use and inquires about the different R values associated with each form.
  • Another participant emphasizes the importance of understanding the variables in the Ideal Gas Law, suggesting that clarity on definitions can aid in addressing the original question.
  • A participant expresses difficulty in identifying the correct R value when using specific volume, noting that their thermodynamics class uses a notation (R bar) that differs from other sources.
  • One participant comments on the variability of notation, stating that while the general form of the Ideal Gas Law (PV=nRT) remains constant, the use of alternative forms may require different R values for different gases.
  • It is noted that if specific volume is used, the second equation can be derived from the first by dividing by the number of moles, but this leads to complications with R values for different gases when mass is involved.
  • Another participant concurs that different gases necessitate different R values when mass is considered, mentioning that this approach is common in atmospheric sciences.

Areas of Agreement / Disagreement

Participants acknowledge the existence of different forms of the Ideal Gas Law and the need for varying R values, particularly when dealing with specific volume and mass. However, there is no consensus on a standardized notation or a definitive method for determining which form to use.

Contextual Notes

Participants highlight limitations related to notation inconsistencies across different educational materials and the dependence on specific definitions of variables, which may affect the understanding and application of the Ideal Gas Law.

Jack Duncan
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Hey I was hoping someone could be me a succinct method of knowing what form of the Ideal gas law I need to use and in particular the different R's associated with each form.
Form my Thermodynamics class we use
PV = nRT
Pv = RT
PV = mRT
Little v being the specific volume (which changes the R value I suppose)

I realize it's a vague question but I'd appreciate any insight.-J
 
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Did you consider what the various variables stand for ? Hyperphysics does. That helps a lot to answer your question.
(Not trying to be nasty: your question IS relevant and it definitely is very important to understand what is all stands for)
 
That is helpful, although taht seems to be simply for the "normal" ideal gas law (at least normal to me) I'm having more difficulty I guess knowing what R is for the ideal gas law using specific volume. In my thermodynamics class right now, it's denoted as R bar (not unit of pressure) and when looking through my book vs online resources or even lecture notes the notation doesn't seem to remain constant
 
I am afraid this part about notation not being standard is something you have to be ready for. Fortunately, it is not notation that matters, but ideas behind.

PV=nRT holds always as written, with R value that is always the same (and identical to the R value listed in every source you will find, and in every equation not related to the ideal gas). Sometimes it looks like it is more convenient to use not the general form of the equation, but some specific forms, in which we can ignore number of moles, or mass of the gas - then we use R' values (where ' I use ' to mean anything different than the standard R) but we can ignore n, or molar mass of the gas. However, it also means in such case we need separate R' values for each gas, so the convenience of that approach is questionable.
 
If v is volume per mole (v=V/n) then the second equation is identical to the first if we divide both sides by n.
As for the third, different gases have different molecular weights so if m is a mass one would have to have a different R for every gas to make it true.
 
Jason2 said:
so if m is a mass one would have to have a different R for every gas

Yep. unfortunately this is true. From what I know this approach is used in atmospheric sciences. Some time ago I tried to read a textbook on climate and I found it quite difficult to follow the formulas in which R was not constant.
 

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