# The Naked Scientists Forum

### Author Topic: How did Avagadro work out the number of atoms in a mole?  (Read 6846 times)

#### don kingsley

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##### How did Avagadro work out the number of atoms in a mole?
« on: 04/07/2008 09:35:17 »
don kingsley  asked the Naked Scientists:

Since you asked for any science questions, here is one that bothers me... Considering what was known at the time, how did Avagaadro compute his estimate of atoms in a mole?

Don Kingsley, Hastings Ne, USA
What do you think?
« Last Edit: 04/07/2008 23:30:38 by chris »

#### cconley

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• Posts: 6
##### How did Avagadro work out the number of atoms in a mole?
« Reply #1 on: 10/07/2008 21:16:42 »
New user and new fan of the show.  Here goes:

Avagado’s number is a bit of a misnomer.  Avagadro did not propose the number.  The number is named in his honor, recognizing his experimentation with gases (Avagadro’s Law).  Avagadro was well dead by the time the number was introduced.

6.02x10^23 is the inverse of the conversion from grams to atomic mass units (amu).

1 amu is equal to about 1.66x10^-24g.

So when one is asked to convert from grams to moles, it is just the opposite from converting grams to amu.

-Chris: CT, USA

#### HellFrozenOver

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• Posts: 12
##### How did Avagadro work out the number of atoms in a mole?
« Reply #2 on: 13/07/2008 11:32:57 »
Hmm.. How did they calculate how much one amu is equal to? If amu is a unit that was manmade how do we know that its really the amount of particles in a mole?

#### cconley

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##### How did Avagadro work out the number of atoms in a mole?
« Reply #3 on: 13/07/2008 15:25:46 »
The mole is also a "manmade" unit based on the amu.  In other words, the amu is the egg that came before the chicken of the mole.

The amu has been defined different ways over time.  Originally, I believe, it was defined as the mass of a hydrogen atom.  It is now defined as being 1/12th the mass of a Carbon-12 isotope.

-Chris: CT, USA

#### Atomic-S

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##### How did Avagadro work out the number of atoms in a mole?
« Reply #4 on: 14/07/2008 06:24:43 »
If you have an electrolytic process that plates out a metal, you can calculate the total charge necessary to plate out one gram. (current times time). From e/m (obtainable from electromagnetic experiments), that gives you the total mass of the electrons involved, but not their count. You can get that, however, if the charge of the electron is known. The charge of the electron was measured by the Millikan oil-drop experiment, in which tiny droplets of oil were sprayed between charged plates and their motion observed to be quantized. The quantization interval was simply the charge of one electron. Once you have that, you now know how many electrons were involved, which, after adjusting for the valancy of the element, gives you the total number of atoms. I do not know if this was the method by which Avogadro's number was found, but it is a possible approach.

#### The Naked Scientists Forum

##### How did Avagadro work out the number of atoms in a mole?
« Reply #4 on: 14/07/2008 06:24:43 »