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What is an AMU Revisited!
Name: Ravi V.
Status: student
Age: 20s
Location: N/A
Country: N/A
Date: 2001
Question:
Dear sir,
I wish to correct the mistake in your answer to the question, "what
is AMU?"
I am reproducing your answers below for reference.
Question: What is the difference between AMU and gram atomic mass?
Thanks.
mark h chu
Answer 1:
The atomic mass unit, or amu, is defined so that the mass
of 1 mole of carbon atoms is exactly 12.000000000 amu.
The amu has the added convenience of being equal to units
of grams per mole; 1 amu = 1 gram/mole. So, one mole of
C12 weighs 12.00000000 grams. I believe that this is strictly
true....someone correct me if I am mistaken.
-dr topper
Answer 2:
The atomic mass unit (amu) is defined to be exactly
1/12 of the mass of a C-12 atom (Not a mole of carbon
atoms...) Therefore 1 C-12 atom has a mass of exactly
12.00000000 amu. Also, therefore, 1 amu is a very
small unit of mass - specifically 1amu = 1.66053X10^-24 grams.
1.00 mole of C-12 atoms has a mass of 12.0000000 grams!
Replies:
Your first answer implies that 1 mole of C atoms = 12 amu.( read line 2
of answer 1).
But the second answer implies that 1 atom of C = 12 amu. ( read line three
of answer 2).
Now what is right?!!!
To be literal, 1 AMU (and that should actually be 1 dalton, to be literal)
is 1.6605402x10^-27
kg (not expressed as grams, to be literal). However, in practice chemists
and physicists convert back and forth between atoms and mols so frequently
and routinely that there is seldom if ever any confusion whether the atomic
or the laboratory scale is the proper context.
Vince Calder
Picky, picky.
The two answers are essentially equivalent. A amu is the mass that an atom
would have if a mole of those atoms had a mass of 1 gram. So, one amu = 1/A
grams, where A is Avogardo's number.
Avogadro's number is defined so that many atoms of carbon-12 (that is,
one mole of carbon-12) would have a mass of exactly 12 grams.
So, you see, if an element has an atomic mass of x amu, it also has an
atomic mass of x grams/mole. So, the amu IS equivalent to a gram/mole.
It is all a matter of the units in which you express it. The first answer is
erroneous only in its first statement - the mass of a mole of carbon atoms
is exactly 12 grams, not 12 amu. Everything else in both answers is
perfectly correct.
Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
As a matter of fact, the carbon-12 atom was
chosen as a basis for relationships between all
other atoms. The mass of a carbon atom was
divided into 12 parts, and each part is known as
atomic mass unit.
12 grams of a carbon atom corresponds to an
gram-atom and contains, according to Amedeo Avogadro,
6,022 x 10^23 atoms. That means, a single carbon
atom has the mass of ca. 1,99 x 10^ -23 grams.
This number, divided in 12 parts (= amu) gives
1,6605 x 10^ -24 g.
Back to your question:
" What is the difference between amu and gram
atomic mass ? " we can say :
1 amu = (1/12) of the mass of a carbon atom = 1,66 x 10^ -24 g
1 carbon gram atom = 12 grams of carbon
In other words, a "gram atomic mass" is the atomic mass
of an atom, taken in grams. Furthermore, the mass of
an atom is mostly concentrated in the nucleus. The carbon
atom has 6 protons and 6 neutrons. If you divide the
mass of an carbon atom (= 1,66 x 10^ -24 g) into 12 parts,
you get - as seen - an amu. You can see also, that a
proton and a neutron have each one 1 amu ( more precisely,
1 proton = 1,0073 and 1 neutron = 1,0087 amu ).
Last but not least, the second answer is correct, that is :
1 atom of C = 12 amu OK !!!
Alcir Grohmann
Ravi,
One mole of carbon atoms has a mass of 12 grams.
Answer (#2) provided by Dr. Brown is correct. The mass of one amu is
1.66053X10^-24 grams. Avogardo's number of atomic mass units has a mass of one
gram. Multiply 1.66053X10^-24 grams by the Avogadro number, the result will be
1 gram.
Regards,
ProfHoff 338
Dear Ravi,
NEWTON is for K-12 students and their teachers, so I do not
normally respond to messages from college-age students.
However, the tenor of your message compels a response.
It seems to me that rather than "correcting the mistake" you
are actually pointing out that our two answers seem to contradict
each other, and I suppose that they do...basically my
definition is not strictly correct, and I apologize to everyone for that
(but I WOULD like to point out that I said in my
original answer that I wasn't too sure). However, both
answers lead to the same operational definition -
a combination of two definitions leading to a third
"definition."
Basically, Dr. Brown's answer is the most fundamentally
correct. A single atom of carbon 12 (including, I believe, its
electrons) is used to define the amu. The amu is defined to be
1/12 of the mass of carbon 12. Similarly, the mole is defined to
be the amount of a substance that has as many elementary
entities as there are atoms in exactly 12 grams of
carbon 12 (Ref: General Chemistry by Hill and Petrucci, 2nd ed., 1999,
Prentice-Hall). These definitions, when combined, lead to
the fact that 1 mole of carbon-12 weighs exactly 12.0000000 grams
and that carbon 12 weighs exactly 12.000000 grams per mole.
Chemists use the identity 1 amu = 1 gram per mole so
extensively that they generally think (loosely) of the amu
along the lines of the definition I mentioned.
prof. topper
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Update: February 2012
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