Avogadro's Number

Please, a gas with an atmoic mass of M does not “weigh” M grams. It has a MASS of M grams. May seem trivial, but it’s important.

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I believe that the article to which you are referring is «What’s the deal with Avogadro’s number? (17-Jul-2001)»
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Since the article is a Staff Report, not a Straight Dope column, this thread is leaving the «Comments on Cecil’s Columns» forum and going to visit my colleague C K Dexter Haven in the «Comments on Staff Reports» forum.

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cwu, in chemistry, the terms “atomic weight” and “molecular weight” are used to refer to mass. Atomic weight is, to be precise, the ratio of the average mass of a chemical element’s atoms to [sup]1[/sup]/[sub]12[/sub] the mass of an atom of the isotope carbon-12. (Encyclopedia Britannica)

Avogadro’s number is the number of carbon atoms in exactly 12 grams of pure C^12, which is ~6.022 x 10^23. Commonly defined as a “mole.” A mole of anything contains Avogadro’s number of those things. It does not refer to mass.

Just a bit of trivia regarding the parting joke in the same column (http://www.straightdope.com/mailbag/mavogadro.html) where the ideal gas law is considered “close enough for government work”.

Back in the days when NASA was NACA and they were world-renowned for their work in designing airfoils and doing precise work in both theoretical and experimental developments, “close enough for government work” was a compliment that meant very, very good.

Later, when people become more familiar with bungling bureaucrats than the scientists at government labs, “CEFGW” became a snide criticism. Of course, current usage rules, but if you make this comment to a 50’s era scientist, they’re likely to hold you to a very high standard.

Son of Dex’s explanation is a good one, but I think part of the answer to Robert C’s question may still be a bit hidden in the reply. Why does the ideal gas law say that a mole of any gas will have the same volume at a given temperature and pressure? That’s because the ideal gas law is based on a model of vanishingly small spheres, i.e., all atoms or molecules are the same, unspecified size. With no size term specified, the size obviously can’t have an effect.

What does matter is the number of these particles and their absolute temperature. These two terms determine the energy and indeed when you write the ideal gas law as PV=nRT, PV has units of energy. Thus pressure and volume trade off with one another. Squish the volume down and the pressure of the system must go up as the energy of our bouncing balls remains constant.

Okay, granted, I attended Navy Nuke Power school twenty years ago. Granted, the coverage of chemistry was quite sketchy. Granted, Son of Dex works with this stuff day in and day out.

But I swear I was told that standard Temperature is 25C.

Is there another engineering discipline that calls Standard Temperature 25C, and I’m misapplying my memory of what I learned?

In solid-state physics, standard temperature is given as 25ºC. But in gaseous-state physics, standard temperature is 0ºC.

So why do I have this dim memory from chemistry class that standard temp is 4ºC?

Something to do with crystal formation in water perhaps?

Anyway, for the purposes of equations, remember to use Kelvins not Celsius. IIRC that would make standard temp 273 Kelvin.


If somebody had said “it’s a conversion factor from grams to AMU’s, and it just so happens that it’s 22.4l of gas at STP” it would have saved me about 16 years of scratching my head (or was that dandruff?).

Reminds me of that electromagnitism thing, “it’s an observed phenomenon, no one knows WHY” would have helped immensely, “it’s too complicated for your tiny mind, but here’s some bullshit to confuse you”, didn’t.

Couple of things:

  1. I beleive that the 4C is because of water density actually being 1.
  2. The standard temperature does depend on what state and branch of science. For the gas laws that would be 273.15 K.

“Why does the ideal gas law say that a mole of any gas will have the same volume at a given temperature and pressure? That’s because the ideal gas law is based on a model of vanishingly small spheres, i.e., all atoms or molecules are the same, unspecified size. With no size term specified, the size obviously can’t have an effect.” - jonk

The ideal gas law actually goes further than this and also assumes that the gas has no charge, thus van der Waals and electromagnetic forces have no effect upon this equation. Also, the gas law does not not state the size of the molecules, because with no size limitations, then the equation is bunk. What if the size of the molecules was assumed to be say… the size of a grapefruit? In actuality, the molecule is assumed to be a point mass. In otherwords, the molecule has a mass but occupies no volume. This is the same thing that is assumed with generic equations for gravity. Finally it (ideal gas law) assumes that all collisions are elastic (no energy is lost in a collision).

But as far as the ideal gas law goes, it is more than sufficent for most applications by the common person. Ignoring specialty chemicals and charged ones, avoiding extreme pressures and temperatures (including ones induced by extreme mass) the ideal gas equation is typically less than 15% off of the true values.

Which electromagnetism thing is that? Electromagnetism is by far the best understood of the forces, so you probably didn’t encounter something that “no one knows why”. It’s much more likely that your teacher either just didn’t understand it himself, or that he was lousy at explaining it.

If you hop on over to General Questions and ask about it, I’m sure that someone here could explain it to your satisfaction.

My first time ever to submit a comment on ANY subject. Delighted to join the Teeming Millions and, perhaps, add some light to the discussion. Hopefully won’t enhance the confussion.

If my time in college was not a complete waste, I recall there are several sets of agreed conditions to refer to when analyzing a given system, in this case a gaseous system. I write agreed because that’s exactly how they came to be, during technical or scientific meetings, and their purpose is to make all communications easily verifiable.

So, there is the standard set of conditions, mainly used in the US and it’s sphere of influence (i.e. commerce), using a temperature of 60 degrees Fahrenheit (some 15.5 C) and one atmosphere of pressure. Then the normal conditions, used elsewhere in the world and especially in science, use zero degrees Celsius and one atmosphere of pressure. And finally, depending on the subject, there are other sets of conditions, an example being the base conditions (20 degrees C, one atmosphere), used in some petrochemical operations.

Did this help?

It’s hard to imagine Avogadro’s number could elicit this much discussion. BTW, in Germany and some other countries, the number goes by the name “Loschmidt’s number”. Loschmidt was a tireless chemist who actually determined its value.

Concerning electromagnetism, on the one hand, it is not a trivial matter that can be handled in a post. I have taken five semesters of classical electrodynamics in undergraduate and graduate work, and there are still areas that are pretty puzzling to me, especially involving radiative phenomena, but also simple magnetism.

On the other hand, “we don’t know why but . . .” is pretty accurate, as far as I can see. Oh sure, by the time we got to QED (quantum electrodynamics) the instructor embedded the phenomena in a shorter equation (the notation hid the complexity – maniacal laughter is heard). But do you really understand something when you write down a Lagrangian?

  1. Why are we including these terms?
  2. What is canonical quantization? Why do we do that?
  3. Why do we use a “Mexican hat” potential? Did physicists not know the word sombrero?
    The answer to all these questions is, “Doing it this way gives the observed phenomena.”