# AMU & Mole

1. Mar 22, 2009

### wajed

what is the relation between AMU & Mole?

also what is the difference between "Molar mass" and "mass"

"It is a common misconception that the mole is defined in terms of the Avogadro constant. It is not necessary to know the number of atoms or molecules which are present in order to use the mole as a unit of measurement,[5] and indeed the first measurements of amount of substance predate modern atomic theory and any measurements of atomic weight"

and I dont get it, what is mole after all? and why is it a misconception to define mole in terms of Avogadro constant? and how does mole exactly "work"?

I know, lots of questions, Im just pretty much confused, even my doctor didn't help me... I brought him a paper, and the first thing he wrote was "mole = molar mass", and I was like, Does mole equal molar mass?! is that an equal sign?! and he said "yes"!

2. Mar 22, 2009

### wajed

scratch the second question, which is "what is the difference between....", I understand that now..

3. Mar 22, 2009

### yougene

A mole is a measurement of an amount of things. Like a dozen or a pair. It's 6.023 * 10^23 particles. You could say you want a mole of eggs or a dozen hydrogens, same type of measurement.

Molar Mass is how much one mole weighs of a particular compound. You can have a mole of hydrogen and a mole of helium but the mole of helium is going to way twice as much because a hydrogen has 1 proton+neutron whereas the helium has 2 proton+neutron pairs.

The AMU is a unit of mass measurement on the atomic scale.

1 mole of 1 AMU weights out to 1 gram. By knowing the AMU of an atom or compound you automatically know its molar mass.

4. Mar 23, 2009

### wajed

but doesnt that contrast with this quote:
"It is a common misconception that the mole is defined in terms of the Avogadro constant. It is not necessary to know the number of atoms or molecules which are present in order to use the mole as a unit of measurement,[5] and indeed the first measurements of amount of substance predate modern atomic theory and any measurements of atomic weight"
??

5. Mar 23, 2009

### alxm

Typical wikipedia. Over-edited into conflating various statements into something that's essentially wrong.

Yes, it's true that historically people used 'moles' that weren't modern 'moles'. And it's true that you can do most chemistry just fine without knowing the exact number of molecules involved - but rather work in terms of proportions; which is what they initially did.

But the modern 'mole' is defined in terms of Avogadro's constant.

6. Mar 23, 2009

### wajed

So whats the difference between "old" moles and "new" moles? giving a keyword that I can use to search in google is enough, if you like

because the standard definition doesn't mention "Avogadros number"

Last edited by a moderator: May 4, 2017
7. Mar 23, 2009

### chemisttree

See http://physics.nist.gov/cuu/Units/current.html" [Broken].

Last edited by a moderator: May 4, 2017
8. Mar 23, 2009

### chemisttree

Not the official definition but certainly the concept is related to Avogadro's number.

Definition:

mole - The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12; its symbol is "mol."

Avogadro's number - The number of atoms or molecules needed to make up a mass equal to the substance's atomic or molecular mass, in grams. It happens to be 0.0012 kilograms of Carbon-12.

9. Mar 28, 2009

### mikelepore

I think you meant to type 0.012 kg ?

10. Mar 31, 2009

### chemisttree

Yes, of course.