atomic weight

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Sach

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Can someone help me out here? I seem to be confusing something...

so I get how carbon-12 atom, = 12 a.m.u (atomic mass units) since it has 6 neutrons and 6 protons for a total of 12 amu's

now I read that 1 a.m.u = 1.66 x 10^-24 grams

Ok fine so far...
but then how come they say on the periodic table one carbon atom = 12 g/mol??? where do they get this from? isn't that just 12 a.m.u

if you take 1.66 x 10^-24 grams (1 amu) and multiply it by 12, you don't get 12 grams..

I am lost, someone help me out here...

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Can someone help me out here? I seem to be confusing something...

so I get how carbon-12 atom, = 12 a.m.u (atomic mass units) since it has 6 neutrons and 6 protons for a total of 12 amu's

now I read that 1 a.m.u = 1.66 x 10^-24 grams

Ok fine so far...
but then how come they say on the periodic table one carbon atom = 12 g/mol??? where do they get this from? isn't that just 12 a.m.u

if you take 1.66 x 10^-24 grams (1 amu) and multiply it by 12, you don't get 12 grams..

I am lost, someone help me out here...

This can be real tricky, but lets keep it simple. First Carbon 12 is the standard from which amu and mole are defined.

Definitions:
1 atom of C-12 = 12 amu
1 mole = # of atoms in 12g of C-12 = 6.022E23 atoms

Now, how many amu make a mole?

a) 12*6.022E23amu = 1 mole = 12g C-12
simplify and you get 1 amu = 1.66E-24g, which u stated already.
Also, divide equation a) by 1 mole and you get 12amu=12g/mole which simplifies to 1amu=1g/mol

what u missed from ur calculation is the mole part. amu=g/mol therefore to get g, multiply amu(1.66E-24g) by mole and voila 1gram:D:D:D:D:D!!!!!

Also 6.022E23amu = 1g, since by definition 1 mole = 12g C-12
 
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Oh this gives you total number of amu's in 1 mole?

remember both amu and moles are defined based on C-12. so i multiplied by 12amu to get total amu/mole..

I had to think through this a bit, it can be tricky.. post more questions if ur not clear..
 
The literal definition of amu is the carbon atom:the mass of exactly 1/12 of the mass of carbon-12. This is just a made up standard of mearurement for atomic masses of all elements.

Amu's on periodic table are averages of all isotopes of that element. Beacuse amu's are small, in chemical equation we use molar mass (g/mol) which deal w/ many atoms not just small amounts.

Avogadro's # is 6.022 X 10^23 experimentally found #of carbon-12 atoms. Molar mass is mass (in grams) per 1 mol. So using carbon-12 as a example: 1 mol of carbon has exactly 12 g and has 6.022 X 10^23 atoms. Really amu= g/mol.

So for example when we need to find # of mols of element X in sample X, grams of sample can easily cancel out g/mol in dimensional analysis. So g/mol is much more useful than amu's in this case.
 
remember both amu and moles are defined based on C-12. so i multiplied by 12amu to get total amu/mole..

I had to think through this a bit, it can be tricky.. post more questions if ur not clear..

you cleared up alot!! thanks..have you taken the MCAT yet?

I guess it became alot simpler when i read amu = g/mol
but in a way I am still confused as to how you get g/mol from amu. I am trying to write it down on a paper and see how i can get g/mol from amu.
 
The literal definition of amu is the carbon atom:the mass of exactly 1/12 of the mass of carbon-12. This is just a made up standard of mearurement for atomic masses of all elements.

Amu's on periodic table are averages of all isotopes of that element. Beacuse amu's are small, in chemical equation we use molar mass (g/mol) which deal w/ many atoms not just small amounts.

Avogadro's # is 6.022 X 10^23 experimentally found #of carbon-12 atoms. Molar mass is mass (in grams) per 1 mol. So using carbon-12 as a example: 1 mol of carbon has exactly 12 g and has 6.022 X 10^23 atoms. Really amu= g/mol.

So for example when we need to find # of mols of element X in sample X, grams of sample can easily cancel out g/mol in dimensional analysis. So g/mol is much more useful than amu's in this case.

1 mol Carbon = 12 grams by DEFINITION?
 
you cleared up alot!! thanks..have you taken the MCAT yet?

I guess it became alot simpler when i read amu = g/mol
but in a way I am still confused as to how you get g/mol from amu. I am trying to write it down on a paper and see how i can get g/mol from amu.

It's straightforward to get amu=g/mol IF you accept the definitions of amu and mole as a precondition. I think it's analogous to understanding electricity by accepting the definitions of charge on an electron, the concept of fields etc.. If you want to derive the definitions by some thought-experiment, i'm afraid i cant help there.

If you accept that:

1 atom of Carbon 12 = 12 amu AND
1 mole = # of atoms in 12g of Carbon 12 = 6.022E23 atoms

Different books can word the definitions differently but they are mathematically equivalent.

Now I want to prove that 1amu = 1g/mol. Since both are defined based on Carbon 12, I'll try to express the 2 equations into one.

1 mole = number of atoms in a 12gram Carbon-12 =Avogadro's number
= (6.022E23 C-12 atoms)
= (6.022E23 C-12 atoms)*(12amu/atom)
Here I'm just converting C-12 atoms to amu

From mole definition,
1 mole = # of atoms in 12g of Carbon 12 = 6.022E23 atoms
this implies that 6.022E23 C-12 atoms = 12g

Now let's use our amu conversion from above:

(6.022E23 C-12 atoms)*(12amu/atom) = 12g
divide through by 12

6.022E23amu = 1g

Remember 1 mole is simply a measure just like we have dozen eggs, billion people you could have a mole of ducks. It would just be 6.022E23 ducks!!

therefore 6.022E23amu = 1mole amu = 1g
divide everything through by 1mole and you get
6.022E23amu/1mole = 1mole amu/1mole = 1g/1mole
1amu = 1g/mole :thumbup::thumbup::thumbup:
 
You are the man :D
Thanks a lot :thumbup::thumbup::thumbup::thumbup::thumbup:

I mean I knew Avagardo's number says in 1 mole, there are 6.022 x 10^23 atoms...I didn't realize 12 grams was included in the definition.
 
you cleared up alot!! thanks..have you taken the MCAT yet?

I guess it became alot simpler when i read amu = g/mol
but in a way I am still confused as to how you get g/mol from amu. I am trying to write it down on a paper and see how i can get g/mol from amu.

About the MCAT question, I'm taking it 4/24/09, how about u?
 
You are the man :D
Thanks a lot :thumbup::thumbup::thumbup::thumbup::thumbup:

I mean I knew Avagardo's number says in 1 mole, there are 6.022 x 10^23 atoms...I didn't realize 12 grams was included in the definition.

Ur welcome, i'm glad i could help. Yes number of atoms in 12g of carbon-12 is 1 mole.

By the way, can you answer a doppler question?
If so the question is "Double doppler shift" under study Q & A.

I posted an answer but i'm not sure if its right since i don't have the book to verify it and the question looks tricky. I want some feedback..

thanks
 
Ur welcome, i'm glad i could help. Yes number of atoms in 12g of carbon-12 is 1 mole.

By the way, can you answer a doppler question?
If so the question is "Double doppler shift" under study Q & A.

I posted an answer but i'm not sure if its right since i don't have the book to verify it and the question looks tricky. I want some feedback..

thanks

sure, i will look at that thread in a minute.

by the way, is that what they mean when they said "Carbon is used as a standard" since they know that 1 mole carbon = 12 g = 6.022E23 atoms, thus amu = g/mol, therefore for every element amu = g/mol? am I thinking the right way?
 
sure, i will look at that thread in a minute.

by the way, is that what they mean when they said "Carbon is used as a standard" since they know that 1 mole carbon = 12 g = 6.022E23 atoms, thus amu = g/mol, therefore for every element amu = g/mol? am I thinking the right way?

I try to simplify things and understand them at a basic level. Think about it, how do we measure things, anything for that matter. Humans measure by comparing an unknown to a known, in other words all measurements are made by comparing the thing we want to measure to a standard. From measuring one's height with a standard (measuring tape) or potency of a drug by comparing it to USP standards. The same is true here, chemists have come up with a measure (amu) based on Carbon 12, to measure the atomic weights of all elements with. So u got it exactly right!!
 
Show that C = 12 g/mol.

Given:
(1) amu = 1.66 x 10^-24 g
(2) C = 12 amu = 12 x 1.66 x 10^-24 g = 1.99 x 10^-23 g
(3) mol = 6.022 x 10^23

Proof:
C = 12 g / mol = 12 g / 6.022 x 10^23 = 1.99 x 10^-23 g = (2)
 
Actually there's no need to prove that 12g of C-12 is equal to 1 mole of C-12 because that's how a mole is defined in the first place. From wikipedia:

"The mole is defined as the amount of substance of a system which contains as many "elemental entities" (e.g., atoms, molecules, ions, electrons) as there are atoms in 12 g of carbon-12."
 
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