aga
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Ideal Gas Law discrepancy
Trying to understand the Ideal Gas Law, so been doing some calculatons to see if i understand it correctly.
PV = nRT
and
V=(nRT) / P
Where :-
R = 0.08205746
P=atm
V=litres
T=Kelvin
At STP, 1 mol occupies nRT/P litres = (1 x 0.08205746 x 293.15) / 1 = 24.05514 litres
Seems OK, but i keep reading that 1 mol of a gas occupies 22.4 litres.
Can anyone see the mistake i am making ?
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eidolonicaurum
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Yes, one easily made and one I often made myself. STP is not necessarily standard! In chemistry, standard temperature is 298K or 25C, while in physics
it is 273K or 0C. I may have mixed the subjects up, but change the temperature and see what you get. 1 mole of gas occupies 22.4 litres at 0C, but 24
litres at 25C.
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unionised
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STP is not the same as NTP.
STP is 0C and NTP is 20C
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BromicAcid
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Indeed, even the Gold Book lists STP as 0C:
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications,
Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8.
doi:10.1351/goldbook.
Last update: 2014-02-24; version: 2.3.3.
DOI of this term: http://goldbook.iupac.org/S06036.html
[Edited on 4/7/2014 by BromicAcid]
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blogfast25
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And you do know just how many atoms or molecules that amount contains, yes?
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aga
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At 0 C (273.15K), 1 atm, the 6.022 x 10^23 atoms (or molecules)  do in deed
occupy 22.414 litres of volume.
Thanks for clearing that up - it was driving me crazy.
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blogfast25
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Always bear in mind that even, say a small shot glass of water (18 g), contains that same number of water molecules. 
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aga
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On to understanding why i can happily have a pot of Copper Nitrate (glad i built a fume hood) next to me, but am too cautious to throw magnesium into
nitric acid.
The metals seems similar given the 1 electron difference in the valence shell, but intuition said that it could be explosive (basically cos magnesium
burns well, and nitric is a strong oxider) which a bit of googling confirmed.
Copper Sulphate was a good place to start - easy to do, and a result that can be used to make other stuff.
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annaandherdad
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Do it in dilute nitric acid and you will be ok (if it's dilute enough).
Any other SF Bay chemists?
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blogfast25
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Magnesium is a very electropositive element: its two outer s electrons can be removed fairly easily, that's what makes alkaline and earth alkaline
elements so reactive, especially when there's something else around that loves absorbing these electrons, like
H<sub>3</sub>O<sup>+</sup> ions in the case of a nitric acid solution.
Almost anything is safe if done on a small enough scale. Dumping ounces of Mg into nitric acid would always be a dumb idea unless you tried it first
on a much smaller scale.
[Edited on 7-4-2014 by blogfast25]
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zed
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Also keep in mind that some gases are diatomic, some gases are mono-atomic, and there may be other possibilities.
Heh, heh. In one of my earliest exams, a diabolical professor concocted a clever gas problem that required complex calculations. Pretty much
everyone got the wrong answer.
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blogfast25
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Quote: Originally posted by zed  | Also keep in mind that some gases are diatomic, some gases are mono-atomic, and there may be other possibilities.
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May be? How about SO2, SO3, SF6, vaporous CCl4, SnCl4, SbCl5, etc etc?
The list goes on and on...
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smaerd
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On a physical chemistry quiz I actually made the mistake between confusing STP(273k,etc) and SATP(298k,etc), zero credit in a typical p chem fashion.
I think that the notation involved in thermochemistry needs some serious revision. There are times where it is significantly easier to follow my
applied partial differential equations book than it is the physical chemistry text (atkins). Ironically I know nothing about partial differential
equations and am pretty inept at mathematical literacy. Once I understand what the p chem text is saying I generally already knew the information or
it could have been stated significantly more easily. Sometimes I wonder if a lot of revisions are held back from the field so that people make dumb
mistakes in academic settings with dimensional analysis or something.
But seriously the very name "Standard state" is almost a joke. There are three standard states and depending on which course or setting you are in
everyone uses them differently (but religiously). Even different definitions use them differently iirc. Supposedly IUPAC uses STP and not SATP, yet in
my p chem course it is all SATP. Other people use NIST (probably the quickest way to find thermo data in the 21st century). Then there's also the
'thermodynamic standard state' which is just whatever the hell conditions a system is/was in. Don't even get me started on the Bar pressure unit, the
subscripts convention involved in chemical thermodynamics, the star-asterisk-knot-nonsense, or how older thermo texts intentionally used awful units
just to make more room for dumb mistakes(rankine inches cubed per millacalorie crap), or what happened to the liter? and how a cubic decimeter in most
situations is not more practical. It's almost funny how something so elegant and beautiful can become convoluted and esoteric.
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zed
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Most specifically Blogfast, I was referring to common gases like Hydrogen and Helium. The noble gasses have completed outer electron shells, and are
thus mono-atomic. Something a novice needs to remember when going from one gas problem to another.
Otherwise, the weight of a specific volume of gas, at STP, can be drastically miscalculated. He is just He.....H is H2.
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Chemosynthesis
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| Quote: Originally posted by smaerd | | On a physical chemistry quiz I actually made the mistake between confusing STP(273k,etc) and SATP(298k,etc), zero credit in a typical p chem fashion.
I think that the notation involved in thermochemistry needs some serious revision. There are times where it is significantly easier to follow my
applied partial differential equations book than it is the physical chemistry text (atkins). |
I also had to use Atkins and feel your pain.
Edit- that said, I have been told it is one of the best p-chem books. Anyone disagree?
[Edited on 10-4-2014 by Chemosynthesis]
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blogfast25
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I don't know about that specific book but Atkins is one of Britain's best chemists and I like his 'Inorganic Chemistry ' (Atkins & Shriver).
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DraconicAcid
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I liked the book when I took it, but my students didn't like the condensed version when I taught it- they would have preferred the full version.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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