How Did Amedeo find the number of molecules present in mole?

[albertlee]

As stated in the topic, how did he find out the numbers as nowdays Avogadro's constant?

 

Albert

[Firedragon52]

Avogadro technically wasn't the first to come across this number:

http://en.wikipedia.org/wiki/Avogadro's_number

 

 

Avogadro’s number is determined by calculating the spacing of the atoms in a crystalline solid through X-ray methods and combining this data with the measured volume of one mole of the solid to obtain the number of molecules per molar volume.

http://www.bartleby.com/65/av/Avogadnmb.html

[albertlee]

then why he was named for this constant?

[Firedragon52]

then why he was named for this constant?

Question

With such limited resources of his time, how did Avogadro come up with his number (the number of molecules in a mole of gas)?

Asked by: Chris Redgate

 

Answer

The short answer is: he didn't!

 

Avogadro stated the theory that equal volumes of gases at the same temperature and pressure contained equal numbers of molecules, being the first to tease out the discrepancy between molecules and atoms. Unfortunately no one listened to him (probably because he was going against the ideas of one of the most revered proto-chemists of his day, Dalton) Avogadro died before anyone looked at his paper and saw the wisdom in his ideas.

 

In any case, someone had to come up with the number of particles in these volumes of gas Avogadro was talking about. Loschmidt, Maxwell, and Kelvin made early estimates of how many molecules could be found in a given volume of gas at standard temp and pressure based on estimates of molecular diameters, the mean free path of a molecule according to the kinetic theory of gases, and some fancy calculation.

 

Plank, Einstein, Millikan and Perrin all tried to use the new mathematical tools presented by quantum theory to refine estimates of how many molecules are in a given volume of gas early in the 20th century. In the meantime, the actual number searched for shifted to the number of oxygen atoms in 16 grams of O, since it was known that Oxygen was 16 times heavier than hydrogen (just lucky that they were both diatomic!) Once atomic structure came to be known, the value sought was understood to be the number of times 1 atomic mass unit can be divided into one gram. (this is often stated as the number of protons in one gram, but 1 AMU is actually closer the mass of 1 proton plus one half of an electron, or 1/12 the mass of an atom of C-12)

 

This number was called 'Avogadro's Number' by Perrin, who wished to honor the man who never received recognition during his life for his substantial contributions to early chemistry.

http://www.physlink.com/Education/AskExperts/ae301.cfm

[albertlee]

thx Firedragon52!!

[albertlee]

ANd more over, Amedeo first said that gas like hydrogen and oxygen was not formed as individual atom, but a single molecule of these, which was a confliction of Dalton's theory during that time, but how does his discovery of molecule and atom associating with his so-called Avagadro's law nowadays? or in another saying, how did he find out about the difference of atom and molecule? (During that time, atom and molecule basically meant the same thing)

 

Albert

[Martin]

Albertlee has raised a bunch of interesting questions. So far the answers are good but not very specific. I am still wondering how, in 1865, the Austrian guy named Loschmidt could have experimentally determined the number.

 

I followed one of the links in this thread and got this:

http://gemini.tntech.edu/~tfurtsch/scihist/avogadro.htm

 

and this says:

--quote--

The first person to have calculated the number of molecules in any mass of substance was Josef Loschmidt, (1821-1895), an Austrian high school teacher, who in 1865, using the new Kinetic Molecular Theory (KMT) calculated the number of molecules in one cubic centimeter of gaseous substance under ordinary conditions of temperature of pressure, to be somewhere around 2.6 x 10¹⁹ molecules. This is usually known as "Loschmidt's Constant." (This value, n_o, is now listed at the NIST web site as 2.6867775 x 10²⁵ per cubic meter)

--end quote--

 

I can easily understand how, if one can measure Loschmidt constant, one can determine Avogadro number. Loschmidt tells me the number of molecules in a cubic centimeter of gas (at normal pressure and temp) and I can just WEIGH a cubic centimeter and divide and then I will know how much each molecule weighs. So then I can easily figure the number of objects in a mole.

 

I can also easily understand the MODERN methods where one uses Xray diffraction to measure the spacing in a crystal and so one estimates the number of silicon atoms in the crystal and then one weighs the crystal and so one knows how much each atom weighs.

 

What is still a puzzle is this---and it is really what albertlee is asking, or so I think---how did someone in 1865---how did this Austrian highschool teacher---count the number of objects in a cubic centimeter of gas?

 

If I was asked I would immediately think of things like the Boltzmann constant and the speed of sound and PV = NkT, and stuff like that. But

Boltzmann constant, IIRC, and the formula for the speed of sound that I am used to, and all that stuff, that comes AFTER 1865

 

so how did he, with such limited resources, figure out the number of things buzzing around in a bottle of air? swansont can probably explain this, or some other local expert. maybe there's a link that actually tells the nitty on this?