Specialty Chemistry Forums > Citizen Chemist

Musings on Ideal Gas Law PV=nRT

<< < (2/3) > >>

billnotgatez:
I continue to muse -

I ran across the below statement on another list having to do with aviation and it made me think about how I hate the term STP.


--- Quote ---ICAO/ISO standard atmosphere is 15 C (59 F), which is more realistic than the oft-used scientific baseline of 0 C (32 F). But in either case the rate of pressure drop with altitude is approximately the same.

--- End quote ---

-----------------------

Some time ago on another thread I did a rant. Here is an edited version


--- Quote ---<rant mode on>

All values in the ideal gas equation must be converted to absolutes. So for instance 273.15 K is the absolute and 0 C is not the absolute temperature. I can not think of a way that you can have a volume that is not absolute. But, I have had problems presented to me that the pressure was not stated as and absolute. I can remember having to convert PSI to APSI (absolute PSI) and then convert to metric or ATM. So when I see a posting having 0 ATM, I assumed that it was not absolute and had to be changed to 1 AATM  (absolute ATM)  for STP. It never occurred to me that the post was a typo. Having said this it seems every post I have seen in the past ATM is assumed to be AATM.

This is the stuff that drives me crazy since STP and ATM are something one is expected to know even though it depends on who you are dealing with when answering the question. I personally think it is poor form to use STP rather than state the temperature and pressure, and also not state pressure as absolute. Many gages do not measure absolute.

<rant mode off>

--- End quote ---

zaphraud:

--- Quote ---This is the stuff that drives me crazy since STP and ATM are something one is expected to know even though it depends on who you are dealing with when answering the question. I personally think it is poor form to use STP rather than state the temperature and pressure, and also not state pressure as absolute. Many gages do not measure absolute.
--- End quote ---

Yeah huh!

Turbo gauges that report "boost" are irritating in that respect; while the good ones do head negative when the driver is off the throttle, a direct report of the manifold pressure would give a much better idea how much fuel-air is being consumed and thus, contributing to power: absolute pressure has a real relationship to an engine's power output. Boost doesn't.

billnotgatez:
Even more musing on the mathematics of the Ideal Gas Law

I have noticed that a common mistake when doing computations involving the Ideal Gas Law is failing to use the correct units. You need the correct units, if you are going to make the mathematics work properly.

The calculations require that the variables be absolute mathematics to work.

Well, for volume (V) that is easy, there is not really a negative volume. So there is an absolute volume rather than relative volume.

And, this holds true for the amount of substance (n). In this real world, you got stuff or you got nothing.

Temperature (T)on the other hand can be measured relative or absolute.
Looking at WIKI
http://en.wikipedia.org/wiki/Temperature
http://en.wikipedia.org/wiki/Celsius
http://en.wikipedia.org/wiki/Kelvin
http://en.wikipedia.org/wiki/Conversion_of_units_of_temperature

Absolute zero
Kelvin 0.00
Celsius -273.15
Fahrenheit -459.67
Rankine 0.00
Delisle 559.73
Newton -90.14
Réaumur -218.52
Rømer -135.90

Again we need to select the correct units to do the calculations. That may mean you have to convert from the temperature units you have to the absolute temperature units. Many people favor Kelvin over Rankine.

I have seen for pressure (P) measurements the old tire gauge is used. Well, most everyday gages start off at zero so that means they started with atmospheric pressure built in to the measurement. So PSIG (pounds per square inch gauge) is the same as approximately 14.7 PSIA (pounds per square inch absolute). In many of the question you get on test, the questions are biased by giving you the absolute pressure. But, sometimes the testers trick you. In any case, if you do not have absolute pressure, convert what you have to absolute.

----------
Now we have everything converted to an absolute units and then we need to select constant (R) to go with those units.
WIKI has a whole list of potential values of the constant.
http://en.wikipedia.org/wiki/Gas_constant
For instance if our units are Litre Atmosphere Kelvin Mole
0.08205746 L atm K−1 mol−1 would be used for the constant (R).

Borek:

--- Quote from: billnotgatez on March 10, 2014, 09:21:21 PM ---I have noticed that a common mistake when doing computations involving the Ideal Gas Law is failing to use the correct units.
--- End quote ---

This problem is not limited only to ideal gas calculations - my bet is that it is the most common mistake repeated on the daily basis in all schools on all continents.

Perhaps with exclusion of the Antarctica.

billnotgatez:
I just noticed on WIKI
http://en.wikipedia.org/wiki/Gas_constant

8.3144621(75)     L  kPa K−1 mol−1
8.3144621(75)    m3  Pa K−1 mol−1

I am now confused as to Pa and kPa having the same result for R
Maybe I need to sit down and do the math

Navigation

[0] Message Index

[#] Next page

[*] Previous page