Pumukli
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Avogadro's number
I was wondering since I heard about the "mol" dimension that how did they knew it was equivalent with 6 x 1023 or so particles.
Was it just an arbitrary decision like "this is the meter standard, this stick is exactly 1 metres long" or was there a bit more science behind it?
We were not thaught in the schools where this constant came from. The teachers just assumed we already knew it was 6x1023 and we could use
it in calculations so everything was fine. At least I don't remember hearing the background story.
Until NOW!
I was searching something entirely unrelated (as is usual) and came across the attached article in the Journal of Chemical Education, from some fifty+
years ago! A brilliant article about the experimental determination of the Avogadro's constant - with a method very much doable for a home chemist,
especially with the now cheaply available electronic gadgets from China!  Also
very educational in the sense that science was not started 20 some years ago with the advent of Internet. The experiment was carried out in the
1930-ies as a school lab project! With the technique of those days and still produced the right answer!
The article also comes with a very interesting "catch" in the end with some science historical relevance!
Worthy reading and highly educational. At least it was for me.
Enjoy!
Attachment: avogadrodetermination.pdf (1.9MB)
This file has been downloaded 236 times
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j_sum1
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Two questions here.
1 about how the constant was first determined.
Like anything of this nature, it was found approximately first and then with increasing accuracy by a variety of methods. We have now reached a point
where we can determine it axperimentally to a level of precision equal to the level of uncertainty of our fundamental units (that is, the uncertainty
on the kilogram). Which brings us to the second question.
2. The question about the redefining of our fundamental units. Veratassium on yt is probably the most accessible accurate descriptor of this process.
We are basically left with a decision: do we have our fundamental units defined by physical objects (eg platinum cylinders) and suffer uncertainty in
our physics constants, or do we define our fundamental units from the physics constants and suffer uncertainty in our measurement of physical things?
Some time ago a decision was made to define the second in terms of the speed of light rather than determine the speed of light in terms of the second.
Likewise the other fundamental units. Defining the kg in terms of Avogadro's constant is the most recent (and the last) fundamental unit to be
redefined.
So, now we can state Avogadro's constant exactly with no imprecision whatsoever.
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MrHomeScientist
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Great post! This inspired me to do some research, and I found a few other ways of experimentally measuring the constant (I'll call it A
here). Here are some more PDFs and a brief summary of each.
I'll call the method posted here the "electrolysis method".
Electrochemical Method:
From: https://mse.engineering.ucdavis.edu/sites/g/files/dgvnsk4451...
Attachment: GAANN-exp-2-Avogadros-Number.pdf (147kB)
This file has been downloaded 395 times
Create a galvanic cell between Cu and Zn strips in dilute CuSO4 solution, connected via ammeter. Measure the average current over a period
of time, then measure the loss in mass experienced by the Cu strip. From this information, A can be calculated. Most likely requires a high
precision scale.
Monolayer Method:
From: http://www.phillysim.org/newmanual/exp21.pdf
Attachment: phpHLaFsy (89kB)
This file has been downloaded 238 times
Create a monolayer of stearic acid molecules on a watch glass full of water. Using the mass of stearic acid required and the area of the watch glass,
A can be calculated. Here's a video I found on this method: https://www.youtube.com/watch?v=gFL46loId0U
I will definitely try to make a video on one or all of these methods. This is very interesting stuff.
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Pumukli
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Well, it seems my memory is not that reliable at all. Because I realized that one of our teachers actually performed the monolayer experiment in
class! Shame on me saying no one showed us a method... 
Thanks MrHomeScientist reminding me to these long forgotten things!
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