Determining Gas Constant, R, for Arbitrary Units - Idea for Program
Jon Noring
Hello,
As an engineer, I'm always faced with having to find the appropriate value
for the ideal gas constant, R, given the units and how R is defined in the
equations.
For example, R can be defined several ways, the most common is from the
ideal gas law:
R(molar basis)= P*V/n*T
where P is the pressure, V is the volume, n is the number of moles, and T is
the absolute temperature. What is the value of R if the pressure units are
in mmHg, V is in cubic inches, n is in lb-moles, and T is in Kelvin?! You
can see that locating or calculating the correct value of R given the units
can be quite a bit of work, especially if the units are highly "mixed" (of
course, why don't we all work in SI units and all will be happy :^) ).
If we know the molecular weight of the gas, then
R(mass basis)= R(molar basis)/M
where M is the molecular weight in the appropriate units.
Also, R can be defined using energy units:
R (mole basis) = E/n*T, where E is the energy, n and T defined above.
(I believe there are also a few other definitions of R that are occasionally
used which I don't recall at the moment - if anybody can remember these, please
let me know.)
Anyway, to make a long story short, I'd like to see software that will give
the proper value of R given the mixed units. One set of software could be
written in C so it could be transported across lots of platforms. I personally
would like to see a Windows 3.1 version fully using the GUI.
For example, in my dream Win3.1 program, I'd like to first of all select
("click on") the form of R (whether in pressure-volume units, energy units,
force-distance units, or whatever, and select whether it is to be in molar
units or in mass units given the molecular weight).
The window will then display a list of candidate units for each unit component
of R. For example, for the pressure we could have a menu of atm, bar, mmHg,
inH2O, psi, etc. For the volume we could have liters, gals, cm^3, in^3, etc.
For temperature we'd have Kelvin and Rankine (don't know of any other absolute
temperature units). For the moles, we can have gmoles, kg-moles, lb-moles,
ton-moles!, etc. Then when all the units have been selected ("clicked"), the
appropriate value of R will be prominently displayed, e.g.:.
R= 0.048215 +/- 0.000013 inHg*ft^3/gmoles*Rankine
This dream package would also allow one to add/delete units to the menu and/or
database, and to change the internal conversion factors (as well as changing
the fundamental value and uncertainty of R, probably in SI units).
If you have the ability to write such a program (I don't), then by all means
do so. And I fully relinquish this idea to whoever will do the work - I have
no interest in earning any money from this idea (to be honest, I don't think
there's much money to be made in such a program). And I really would prefer
a Win3.1 version as I describe above. If you would like to write such a
beast, I'd be more than happy to help in the formulation of the program
(all I ask is that I be provided the final version free-of-charge if it will
end up being shareware, or heaven-forbid, payware).
Also, if you have any other ideas to make this package even more useful, please
let me know (all ideas submitted to me, though, will be considered non-
proprietary unless you are planning to write the software.)
Any takers out there?
Jon Noring
(what's the value of R given megapascals, cubic furlongs, degrees Rankine,
and lb-moles? :^) )
(In my CRC handbook, 58th edition (an old one), it shows from CODATA bulletin
number 11 the accepted value of R being 8.31441 +/- 0.00026 J/g-mole*K. Has
the value and uncertainty of the Gas Constant been updated since then?)
--
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If you're dying to know what INFJ means, be brave, e-mail me, I'll send info.
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Tomas Eriksson
...various things, shocking for an SI fanatic like myself, deleted...
>(In my CRC handbook, 58th edition (an old one), it shows from CODATA bulletin
>number 11 the accepted value of R being 8.31441 +/- 0.00026 J/g-mole*K. Has
>the value and uncertainty of the Gas Constant been updated since then?)
Yes. From my "Quantities, Units and Symbols in Physical Chemistry",
1988 Edition (the latest), the recommended value of R is
8.314510(70) J K^-1 mol^-1,
where (70) is the uncertainty in the last digits. This value is quoted
from E. R. Cohen and B. N. Taylor, "The 1986 Adjustment of the
Fundamental Physical Constants", CODATA Bull. _63_ (1986) 1-49.
The value of the Avogadro constant is 6.0221367(36)*10^23 mol^-1, by the
way.
Tomas Eriksson (tom...@physchem.kth.se)
Dept. Physical Chemistry, Royal Institute of Technology, Stockholm, Sweden