# Chemistry question

Discussion in 'General Science' started by Fatslug, Oct 18, 2008.

1. ### Fatslug Thread Starter New Member

Oct 11, 2008
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the question, its actually extra credit on my chem test!

"in View of the assumptions made in the ideal gas law ,what quantities would you use instead of the measured volume and pressure in order to make the law give more correct results."

only thing i could come up with was , but i really don't know what i means.

adiabatic, the assumption used to make it easier to solve. it takes terms out

2. ### beenthere Retired Moderator

Apr 20, 2004
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Your faith in us is touching - this is an electronics forum. Someone may be able to answer, though.

Feb 24, 2006
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4. ### Ratch New Member

Mar 20, 2007
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Fatslug,

Use a monotonic gas, low pressure and high temperature. Anything to get away from intermocular attractions, which cause van der Waals effects to come into play, and cause deviation from the "ideal" gas law.

Ratch

5. ### triggernum5 Active Member

May 4, 2008
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If this is a 1st year university or lower class then I'd say Ratch is right, but it almost sounds by the wording that they want you to improve the IGL.. P, V, & T are the simplest values to measure, but there are other qualities that could be derived with more difficulty that may cancel out the non-ideal descrepancies somewhat.. Things like average separation distance, particle velocity perhaps.. Seems a little hefty though, I doubt its reasonable to ask a student to revamp basic chemistry theory in their spare time after a test..

6. ### KL7AJ AAC Fanatic!

Nov 4, 2008
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I would say the mass of the gas would be the most absolute...though difficult to measure.

7. ### jpanhalt AAC Fanatic!

Jan 18, 2008
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From Wikipedia: PV=nRT where n=number of moles of gas, i.e., mass/molecular weight. I am not sure how the accuracy of that equation would be improved by measuring the mass of the gas.

Apart from that, I agree with triggernum5. Applying statistical thermodynamics to the system to consider such aspects as collisional cross sections (probability of collisions), mean free paths, etc. would get pretty complicated, a least for me.

John

8. ### steveb Senior Member

Jul 3, 2008
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I think the key word there is "measured". If you do an experiment, you will measure the volume in a straightforward way and will probably measure the pressure at the container boundary.

However, if you look at typical corrections to the ideal gas law, you will find that the effective volume should subtract off the volume of the atoms themselves from the volume of the container. Also, the pressure on the interior of the container is more accurate since the Van der Waals forces cancel, unlike at the surface.

The problem does not say you need to have a practical formula that uses variables that can be easily measured. It only asks; "what would you use in place of the measured pressure and volume to get more accurate results?".

9. ### triggernum5 Active Member

May 4, 2008
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I'm still leaning to the fact that the question was just horribly worded.. It almost seems like they meant to ask "What type of values for PVT would work nicely with the IGL?".. Ratch answered that.. Never in my chemistry courses was I ever asked to take the place of a super-computer for modelling purposes..

10. ### studiot AAC Fanatic!

Nov 9, 2007
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Surely we are referring to the 1873 version of Van der Walls equation.

He argued that the actual size of the gas molecules reduces the volume available for the gas to move about in
so he replaced V by (V-b), where b is a constant.

He also argued that the attractive forces between the molecules lowers the pressure by lowering the impact force and frequency of the molecules bouncing about. The aggregate of these impacts gives rise to the observed pressure. Both these effects are proportional to the gas density, so he replaced p by (p+a/V$^{2}$), where a is a constant.

These are the quantities I would offer as the answer to the question. The constants a and b have to be obtained by fitting the equation to known P,V and T values.

(V - b)(p + a/V$^{2}$) = RT

11. ### triggernum5 Active Member

May 4, 2008
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I think studiot wins..
(If n=1)

12. ### steveb Senior Member

Jul 3, 2008
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I didn't know it was a contest. But to be fair, I basically said the same thing first, so I should win. I admit I stumbled across the finish line in tattered clothing, and studiot sprinted through in perfect form, but I was first. LoL

13. ### triggernum5 Active Member

May 4, 2008
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Twas a joke.. Boy did I feel stupid when I was perusing my first year notes and realized I've used that equation myself a number of times..
For some reason, even that I knew I had covered that, my mind was telling me there were vast further complexities (which I guess there still are) if you need an even more accurate model..
Anybody know just how well this actually improves on the IGL? I realize its gas dependant, but are we to expect values much closer to experimental with a gas like say SF6?

14. ### steveb Senior Member

Jul 3, 2008
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Of course. I was joking too. I hope I didn't sound serious. Sometimes these smiley faces don't convey the whole meaning.

Unless AAC starts offering prizes for answering, I won't be arguing over who was first, or best.

15. ### zdarken New Member

Feb 14, 2009
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HELLO, the true is that I don't know where to ask.
I work in Electrochemical, and i have a problem, when i try to view a equivalent circuit of a impedance curve with ZVIEW (demo version), this show me a wrong circuit, with wrong values of capacitance. PLEASE HELP ME i need the full version of zview o another solution. THANKS