# Chemistry Relative atomic masses

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#1
The definition for relative atomic mass is;

The weighed average mass of an atom of an element compared with one twelfth the mass of an atom of carbon twelve

but there can be elements that have no isotopes, so you wouldnt apply this definition to it (because an average isnt needed).

The relative molecular mass is the weighed average mass of a molecule of a element or a compound compared with one twelfth the mass of an atom of carbon twelve

how can you get an average for a molecule???

can someone claify these points to me.. thanks
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16 years ago
#2
Add the averages of atoms in that molecule...
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#3
thanksxx
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16 years ago
#4
(Original post by SIMZZ)
but there can be elements that have no isotopes, so you wouldnt apply this definition to it (because an average isnt needed).
to my knowledge, I dont think there are any elements without isotopes. but if there were, you would still apply the defenition, even though an average isnt needed, the average of something that is 100% is itself.
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#5
i get ya, thanks, because if something weighed one gram, you would divide it by 1 and get 1. Thanksxx
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16 years ago
#6
(Original post by SIMZZ)
The relative molecular mass is the weighed average mass of a molecule of a element or a compound compared with one twelfth the mass of an atom of carbon twelve

how can you get an average for a molecule???
Because the molecule may contain isotopes, and therefore the Molecular mass isn't always going to equal exactly what you calculate using the standard figures for RAMs.
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#7
(Original post by Helenia)
Because the molecule may contain isotopes, and therefore the Molecular mass isn't always going to equal exactly what you calculate using the standard figures for RAMs.
yep i understand, for example nitrogen 14 and nitrogen 15. So can you use a mass spectra of a molecule to work out the relatve molecular mass of a molecule?
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16 years ago
#8
(Original post by SIMZZ)
yep i understand, for example nitrogen 14 and nitrogen 15. So can you use a mass spectra of a molecule to work out the relatve molecular mass of a molecule?
yes. that's what they're there for
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