# Forms of the Ideal gas law

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1. Mar 7, 2017

### Jack Duncan

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.

Best,
-J

2. Mar 8, 2017

### BvU

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)

3. Mar 8, 2017

### Jack Duncan

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 doesnt seem to remain constant

4. Mar 8, 2017

### Staff: Mentor

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.

5. Mar 8, 2017

### Jason2

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.

6. Mar 8, 2017

### Staff: Mentor

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.