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Topic: Charles' law  (Read 1973 times)

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Offline Big-Daddy

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Charles' law
« on: November 08, 2013, 06:30:10 PM »
The densities of air at -85°C, 0°C and 100°C are 1.877, 1.294 and 0.946 gdm-3 respectively. Assuming that air obeys Charles' law, determine a value for the absolute zero of temperature in °C.

I know Charles' law - basically the ideal gas equation for a single gas, except that pressure is considered constant to find the volume/temperature ratio - but how does it apply to this question?

Offline Corribus

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Re: Charles' law
« Reply #1 on: November 08, 2013, 07:02:03 PM »
A real crude approximation would be that at absolute zero, the gas has essentially no volume at all. 
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline Big-Daddy

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Re: Charles' law
« Reply #2 on: November 08, 2013, 08:17:40 PM »
Hmm ok, I'm not sure how to use that fact though (which I take note relies heavily on the pressure remaining constant with temperature ...). I did find a completely different way of solving this problem that didn't require setting any V to 0 but I have a feeling you are on the (/a) right track here, can you explain?

Offline billnotgatez

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Re: Charles' law
« Reply #3 on: November 08, 2013, 08:26:29 PM »
@Big-Daddy
This is a real interesting problem to me.
Will you eventually be getting the expected answer from whence you got the question.
regards
Bill

Offline Corribus

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Re: Charles' law
« Reply #4 on: November 09, 2013, 12:52:48 AM »
Hmm ok, I'm not sure how to use that fact though (which I take note relies heavily on the pressure remaining constant with temperature ...). I did find a completely different way of solving this problem that didn't require setting any V to 0 but I have a feeling you are on the (/a) right track here, can you explain?
First, convert your densities to volumes.  Really you can probably just take the inverse, but I used a basis of 1 gram.  Plot your volumes as a function of temperature, do a simple linear regression, find the intercept where volume approaches 0.  I got a value of about -273.2 °C or something.  Closer than I thought it'd be, actually.  Neat problem, I have to say.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline Big-Daddy

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Re: Charles' law
« Reply #5 on: November 09, 2013, 10:58:58 AM »
The answer is given rather vaguely as -272°C. My answer was -273.32°C average across the data points, but unlike Corribus I did not go as far as linear regression, so you are probably more right.

Thanks for clarifying the method.

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