Relationship between amu and g/mol

[MedGrl@2022]

One amu is one 12th of a carbon atom which is assigned a mass of 12.000 (units not given in Kaplan... but I assume g/mol). They say that 1 amu is equal to approximately 1.66*10^-24 grams.

On the next page though it appears to say that 41.15 amu=41.15 g/mol.

I just need some clarification.

Thank you!

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[shades_children]

The mass of a carbon atom is pegged at 12.000 amu, not g/mol. Therefore, one amu is 1/12 the mass of a carbon atom.

I have no clue about "41.15 amu=41.15 g/mol." That's absolutely not true.

 

[bostampa2007]

see if this helps:

1 mol of C has a mass of =12 gm

and 1 mol= 6.023X10^23 atom

so, 1 C atom has a mass of= 12 gm/ 6.023X10^23
=1.9X10^23 gm

Now, according to the definition, 1 amu equals to the 1/12 of the mass of one C atom.

so, 1 amu= 1.9X10^23gm/12
=1.66X10^24 gm

this is why, 1 amu=1.66X10^24 gm

now, you can also say, one C atom has a mass of 12 amu,b/c 1amu is 1/12 of the mass of one C atom.

essentially, when you say, one atom of a certain element has a mass of 41 amu , it means that its atomic mass (neutron+proton) is 41.

hope that helps.

One amu is one 12th of a carbon atom which is assigned a mass of 12.000 (units not given in Kaplan... but I assume g/mol). They say that 1 amu is equal to approximately 1.66*10^-24 grams.

On the next page though it appears to say that 41.15 amu=41.15 g/mol.

I just need some clarification.

Thank you!

 

[themule]

Tampa,

Good explanation but you dropped some negative signs in your exponents. Just sos you know.

TM

 

[MedGrl@2022]

see if this helps:

1 mol of C has a mass of =12 gm

and 1 mol= 6.023X10^23 atom

so, 1 C atom has a mass of= 12 gm/ 6.023X10^23
=1.9X10^23 gm

Now, according to the definition, 1 amu equals to the 1/12 of the mass of one C atom.

so, 1 amu= 1.9X10^23gm/12
=1.66X10^24 gm

this is why, 1 amu=1.66X10^24 gm

now, you can also say, one C atom has a mass of 12 amu,b/c 1amu is 1/12 of the mass of one C atom.

essentially, when you say, one atom of a certain element has a mass of 41 amu , it means that its atomic mass (neutron+proton) is 41.

hope that helps.

I am not sure I follow... gm=grams/mole right? So then why did you divide 12g/mol by 6.02X10^23 atoms. Wouldn't you want to multiply it by 6.02X10^23 moles to get 7.22X10^24 grams. When I divide this number by 12, I still get 6.02X10^23 grams.


However, if I 1.9X10^23gm/12= 1.58X10^23 not 1.66X10^24 gm.

What am I missing?

 

[BerkReviewTeach]

I am not sure I follow... gm=grams/mole right? So then why did you divide 12g/mol by 6.02X10^23 atoms. Wouldn't you want to multiply it by 6.02X10^23 moles to get 7.22X10^24 grams. When I divide this number by 12, I still get 6.02X10^23 grams.


However, if I 1.9X10^23gm/12= 1.58X10^23 not 1.66X10^24 gm.

What am I missing?

It's a technicality really. Carbon-12 is assigned an atomic mass of 12.00 amu, setting the definition of an amu as 1/12 of the mass of the carbon-12 isotope. You can also think of it as the average of the mass of a proton and neutron (given that carbon-12 has 6 protons and 6 neutrons).

However, when we consider a mole of carbon, there is about 1% carbon-13 and a trace amount of carbon-14 (which ironically do not have masses of 13 amu and 14 amu respectively, because they each have more neutrons that protons).

One mole of carbon atoms has a mass of 12.011 grams, because of the presence of isotopes and because and amu is not exactly the mass of a proton or neutron. When everything is said and done, the difference between an amu on the atomic scale and the g/mole on the mole scale is irrelevent, unless you are looking at mass loss or mass gain associated with nuclear reactions.

 

[chromags]

I think you are asking how grams are derived from the AMU unit listed under each element on the periodic table.

Take Lithium for example, whose AMU is 6.94.
By convention, 1 AMU = 1.66 x 10^(-24)
SO take 6.94 AMU and multiply by 1.66 x 10 (-24)

This yields 11.52 x 10 (-24) grams per molecule of lithium
(because of some weird carbon rule which I have yet to master.)

Now you have grams. A "mole" is a magic number that makes any Amu **CONVERTED TO GRAMS*** equal its AMU in grams. But first you need to obtain the grams from the AMU, like we just did for lithium.

So, 11.52 x 10 (-24) grams lithium x 6.022 x 10^23 (avagadros #) =
69.37 x 10(-1) = 6.937 grams = Lithiums AMU = 1 mole Lithium

 

[SN2ed]

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