Moles and AMUs and Na!!!!

[MCATMadness]

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on page 421 of TPR Physical Sciences Review... there is a discussion about how moles, amus, and avogardros number relates.... i am having a hard time understanding the concept. anyone else get it?

i understand the basics about moles, amus, etc. just dont see how it all relates.

 

[DougTPR]

Lemme try to put all those relationships succinctly...

1 atom of carbon = 12 amu

There are 6e23 atoms in 1 mole

There are 6e23 amu in 1 gram

Therefore...

1 mole of carbon = 12 g

In essence, the amu is defined so that there is an Avogadro's number of amu units in 1 gram just as there is an Avogadro's number of atoms in 1 mole.

Hope this helps.

 

[MCATMadness]

Thanks!

One follow up question - did you mean to say "Avo number of amu units in 1 gram"...

so the mole is simply defined to convert between the atomic world (amus) and the micro/macro world (grams). right?


thanks for the help!

 

[Rabolisk]

A mole is a unit to describe amount of substance.

The Avogadro number is the number of particles that exist in 1 mole of substance. It is analogous to saying that 12 is the number of months in a dozen of months.

The numerical value of avogadro's number is 6.022E23. This means that one mole of A consists of 6.022E23 particles of A.

The mass of 1 atom of carbon-12 is 12 amu by definition. Thus 1 amu is 1/12th of the mass of 1 atom of carbon-12.

Now the link between all the units: 1 mole of carbon-12 weighs 12 grams. 6.022E23 atoms of carbon-12 weigh 12 grams, and 1 atom weighs 12 amu. Hence 6.022E23 = gram/amu.

 

[BestDoctorEver]

Lemme try to put all those relationships succinctly...

1 atom of carbon = 12 amu

There are 6e23 atoms in 1 mole

There are 6e23 amu in 1 gram

Therefore...

1 mole of carbon = 12 g

In essence, the amu is defined so that there is an Avogadro's number of amu units in 1 mole just as there is an Avogadro's number of atoms in 1 mole.

Hope this helps.

Sorry if I highjack this thread...I still can't make the connection between like there are 6e23 amu in 1 gram.... Isn't that amu stands for 'atomic mass unit'. I had problem with that when I was taking gen chem and I still have problem with it preparing again for the MCAT.

 

[Rabolisk]

The atomic mass unit is defined as 1/12 of the mass of 1 atom of carbon-12. Put another way, 1 carbon-12 atom weighs 12 amu. (Here I'm using the term weigh loosely)

The mole is a unit for amount of substance, and 1 mole of something contains 6.022E23 "entities" of that thing. The Avogadro's number 6.022E23 is the number of atoms in 12 grams of carbon-12. Thus the definition of a mole is the amount of substance containing the same number of entities as there are in 12 grams of carbon-12.

1 atom of carbon-12 weighs 12 amu, and 1 mole of carbon-12 weighs 12 grams. 1 mole = 6.022E23 atoms. Thus 6.022E23 atoms of carbon-12 weigh 12 grams. Dividing 12 grams by 6.022E23 gives 12 amu, which means that 1 gram = 1 amu * 6.022E23. Hope that helped.

 

[BestDoctorEver]

The atomic mass unit is defined as 1/12 of the mass of 1 atom of carbon-12. Put another way, 1 carbon-12 atom weighs 12 amu. (Here I'm using the term weigh loosely)

The mole is a unit for amount of substance, and 1 mole of something contains 6.022E23 "entities" of that thing. The Avogadro's number 6.022E23 is the number of atoms in 12 grams of carbon-12. Thus the definition of a mole is the amount of substance containing the same number of entities as there are in 12 grams of carbon-12.

1 atom of carbon-12 weighs 12 amu, and 1 mole of carbon-12 weighs 12 grams. 1 mole = 6.022E23 atoms. Thus 6.022E23 atoms of carbon-12 weigh 12 grams. Dividing 12 grams by 6.022E23 gives 12 amu, which means that 1 gram = 1 amu * 6.022E23. Hope that helped.

Thanks..It makes sense now.

 

[DougTPR]

One follow up question - did you mean to say "Avo number of amu units in 1 gram"...

so the mole is simply defined to convert between the atomic world (amus) and the micro/macro world (grams). right?

Yep, I did. And I edited the original post to fix it.

And that is a great way to think about the mole!

And Rabolisk is right that technically the amu is defined via the carbon-12 atom. It sure is nice when numbers work out real nice 🙂 especially since we make it that way.

 

[MCATMadness]

thnx everyone for the help. its funny how weve all worked with these basic concepts for years, but when asked to really think about the relationship and definition....... we struggle. thnx for the clarification!