# Why is the Mr of Carbon 12.0000000

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#1
From an AQA Chem 2 question

State why the Ar of Carbon is taken to be 12.0000000?

Any ideas? I thought it might be because Carbon is the only element with a precise mass?
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10 years ago
#2
because they needed something to base the rest on so they choose carbon
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10 years ago
#3
It's not 12.00000000. It's 12.01?
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#4
(Original post by mobius323)
It's not 12.00000000. It's 12.01?
Yeah, I don't know why but for AQA they reckon it's dot on 12
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10 years ago
#5
Haha, I so read that as 'Why is the Mister of Carbon 12.000000'.
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10 years ago
#6
(Original post by wibletg)
Yeah, I don't know why but for AQA they reckon it's dot on 12
The reference is the carbon-12 isotope = 12.000000

All other masses are relative to that specific atom

Carbon itself is a mixture of isotopes, which have an average of 12.01
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10 years ago
#7
(Original post by mobius323)
It's not 12.00000000. It's 12.01?
My uni article I used for a lab report said 12.0000000 (can't remember how many decimal places it was)

Surely its something to do with a molecular unit being 1/12th of a carbon-12 atom?
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10 years ago
#8
they define one atom of carbon 12 to be exactly 12 so that they can assign numbers to the others relative to this. because using the actual masses would make the maths ridiculous. the average mass of carbon isn't exactly 12 because not just carbon 12 exists other isotopes do to
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10 years ago
#9
(Original post by wibletg)
Yeah, I don't know why but for AQA they reckon it's dot on 12
Carbon-12 is used as the basis of relative masses of other atoms and molecules, so maybe that's why they say it's 12.

But yeah, it couldn't possibly be exactly 12. I assume that AQA must have mentioned Carbon Dating, which uses Carbon-14, at some point in your time of studying?
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#10
(Original post by mobius323)
Carbon-12 is used as the basis of relative masses of other atoms and molecules, so maybe that's why they say it's 12.

But yeah, it couldn't possibly be exactly 12. I assume that AQA must have mentioned Carbon Dating, which uses Carbon-14, at some point in your time of studying?
Nope just carbon 12
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10 years ago
#11
(Original post by mobius323)
It's not 12.00000000. It's 12.01?
Thats the Ar of Carbon (all the isotopes) not Carbon-12. It's the way it's defined as above posters said.
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10 years ago
#12
I believe it is because carbon-12 is the standard on which the system is based, defined by the fact that the Mr of a neutron or proton is 1/12th of carbon-12. I could be wrong though.
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10 years ago
#13
(Original post by Loz17)
My uni article I used for a lab report said 12.0000000 (can't remember how many decimal places it was)

Surely its something to do with a molecular unit being 1/12th of a carbon-12 atom?
Using my periodic table, it says 12.01. And also, when you say "Mr of Carbon", you're thinking the mass of a single Carbon Atom. Therefore, it could either be 12, 13 or 14 (plus more).
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10 years ago
#14
because everything is relative to Carbon. there are 12 electrons, each weighing 1.
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10 years ago
#15
(Original post by Farseer)
Thats the Ar of Carbon (all the isotopes) not Carbon-12. It's the way it's defined as above posters said.
Ar of Carbon is 12.01, yes. But Mr of "Carbon"? That can't possibly be a single number if isotopes are present...
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#16
Sorry, I've just found the precise question

State why the precise relative atomic mass for the 12C isotope is exactly 12.00000

Edit: confusingly, the answer is 'by definition'. Hmm.
1
10 years ago
#17
(Original post by mobius323)
Using my periodic table, it says 12.01. And also, when you say &quot;Mr of Carbon&quot;, you're thinking the mass of a single Carbon Atom. Therefore, it could either be 12, 13 or 14 (plus more).
This is only A level. I'm certain the 12.01 is to account for isotopes. But if you're using one Carbon-12 atom then it will be 12. I need to look more into it (and no doubt I will sooner or later) but since carbon-12 is the most common isotope I think that was used as a rekative mass for all other atoms.

Though if I am wrong, I am exceedingly tired. That is why.
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10 years ago
#18
(Original post by charlotteboulton28)
because everything is relative to Carbon. there are 12 electrons, each weighing 1.
No, for the love of God, no. Electrons weigh 1/1838, or something like that (trying to remember A-level Chemistry). In any case, there are only 6 electrons in carbon. It's the protons and neutrons that weigh one each.
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10 years ago
#19
1 atomic mass unit is defined to be 1/12th the mass of a 12C atom. Everything is measured in atomic mass units, so 12C is therefore defined to weigh 12 amu. The number on your periodic table is the average mass of all isotopes by % abundance, which seeing as all isotopes of carbon weigh 12 or more amu, has to be over 12.
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10 years ago
#20
One past paper question I saw was 'Why is the Mr of Carbon 12 12.0000000' and the answer was 'by defintion' or because Mr is relative to Carbon 12.
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