Subject: Re: CCL:Avogadro's number

From: Peter Shenkin <[email-masked]>
Date: Fri, 12 Mar 1999 13:23:09 -0500 (EST)

On Thu, 11 Mar 1999, Chris Harwell wrote:
> On Thu, 11 Mar 1999, Anatoli Korkin wrote:
> > Last time I was visiting my dentist, he asked me how Avogadro
> > could come with his number at that time. 
> i think he only came up with the idea of a mole and relatvie proportions.
> 
> someone else actually came up with the number after boltzmann & maxwell
> came up with the necessary stat. mech.

Sigh.  What do they teach in freshman chemistry these days?

Based on the observation that gasses combine chemically in the
ratio of small whole numbers, Avogadro *hypothesized* that equal
volumes of gasses (at the same T & P) have the same number of
molecules.  I believe this was around 1811.

Dalton opposed this view, because an observation like
1(volume of H) + 1(volume of Cl) -> 2(volumes of HCl)
would then imply that H and Cl are diatomic, which seemed 
preposterous to him.  Of course, we now know that Dalton
was wrong in this.

But measurements weren't so good then, and there was room
for disagreement.  It wasn't until Canizzaro did better experiments
in the 1840's that Avogadro's hypothesis was accepted.

Still, nobody knew what the number was.  In the late 19th century,
people referred to Loschmidt's number, which was something like the
number of molecules in a gram of hydrogen gas.  (I forget the
exact definition -- it was something like this.)  It could
be approximated from the kinetic theory of gasses by comparing
several measurements;  this gave the right order of magnitude.
This is what Chris is referring to, I think (though I wouldn't
call it stat mech).

Getting a precise value of Avogadro's number was a hot topic
in experimental physics around the turn of the century.  Einstein's
work on Brownian motion (1905) provided the most precise values
to date, AFAIK.

I'm not sure when the quantity we refer to started to be called
Avogadro's number, but it was probably during the late 19th century,
after Avogadro's death. (He died in 1856.)

If I recall correctly, Mahan's freshman chemistry text goes into
a lot of the above history (especially the Avogadro/Dalton/Canizzaro
part).

	-P.

P.S.  Here's a homework problem.  What does a gram of any ordinary
matter contain Avogadro's number of?

--
*********** Peter S. Shenkin; Schrodinger, Inc.; (201)433-2014 ************
*********** [email-masked]; http://www.schrodinger.com ***********

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From: Jochen Kuepper <[email-masked]>
Date: Tue, 19 Apr 2135 04:42:47 +0100

>matter contain Avogadro's number of?

amus, not to say amusement :-)

Cheers