Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question concerning Ideal Gas Law correction to a teacher & tactons

  1. Feb 6, 2009 #1
    Whoops, I meant to say tact. Hehe, maybe tactons are the gauge bosons for the tact force.

    My Chemistry teacher, who has a Ph.D in Biology., is teaching us that one can use the Celsius temperature scale for the Ideal Gas Law using (approximate)[tex] R = 0.0821 \frac{atm * L}{mol * K} [/tex]. Where R is the Universal Gas Constant.

    Now using the conditions of an ideal gas at STP, I attempted to solve for R. Next using possible conditions and the Kelvin scale, I solved for a complete set of conditions. Then I converted temperature to Celsius. I think R is not constant with the Celsius scale.

    Now, if I'm not mistaken, Celsius and Kelvin have the same measure but are offset, so there shouldn't be any problem solving for change in one parameter, but there is no way to solve for one of the parameters absolutely using Celsius.

    Am I correct? How should I tell him this without pissing him off?
    Last edited: Feb 6, 2009
  2. jcsd
  3. Feb 7, 2009 #2
    I spent a couple of minutes looking at this, trying to work out if some approximation is being made (biologists generally only work in regimes of 0-100 C). However I can't think of one.

    The problems are that if you get negative temperature you'll get negative pressures/volumes/mols of gas. Also at 0 C the equation breaks down.

    As for tact, he is your teacher, as a student it is your job to ask questions. So best thing is to is first say 'I don't understand it - can I read about it in a book somewhere?'. I don't see why a teacher would be pulling values out of thin air...

    If there isn't anything (alarms would have to be ringing for the teacher if he can't find anything) ask about the breakdown of the equation at T=0. You have to be genuinely open-minded and curious, trying to 'prove people wrong' is a bad idea as if you attack people they'll go on the defensive and might just entrench their ideas (good people don't but these are kind of rare).
  4. Feb 7, 2009 #3
    I've read that one must have absolute temperature to use the Ideal Gas Equation. He did mention the breakdown at values less than or equal to 0 C, so he may not be communicating to the students that one has to use Kelvin if one is not to use some weird conversion.
  5. Feb 7, 2009 #4
    If you want to plug in Celsius for Kelvin in that equation then instead of using a straight value of K you would use (C+273.15). It will NOT work if you just plug in values of C.
  6. Feb 7, 2009 #5
    The ideal gas law in the form PV = nRT is only valid for absolute (Kelvin) temperatures. Different temperature scales will require a different equation.
  7. Feb 7, 2009 #6
    I now know that I'm right. Now comes my concern over the rudeness of an undergrad correcting a Ph.D. I suppose they're human too, but it just seems like I need to be careful. This prof will probably teach all my remaining chem classes, so I very much wish to be on good terms with him.

    In defense, he's a biologist with an undergrad in chem, so he's more concerned with organic chem than physical chem.
  8. Feb 8, 2009 #7
    To clarify this teacher is not using PV=nRT, he is using rPV=nRT (r is some other constant).

    However as fedaykin said, this is still not valid (well r is also a variable so it's only valid at one point).

    The thing confuses me is the 0.0821 factor. fedaykin, whit I'd do is - ask a question about it. Like 'I've been trying to work out where this 0.0821 factor comes from, have you got a reference?'.

    Then if you still get nothing ask about some examples. Like 'I calculated the rise in temperature you need to double the pressure (at constant vol and mass of gas), then I did the same calculation with the Celsius equation...they don't seem to match. What did I do wrong?'

    As I said if you go in with the attitude that you are 'correcting him' or 'proving him wrong' a bad outcome is more likely. It's better to have the attitude that he is doing something that is correct that you don't understand, and you want to understand it. There is a chance that he is talking about some approximation and has just got it a little wrong!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook