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gctso

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A conceptual problem about mass of atoms
« on: January 28, 2015, 08:45:52 PM »
As I know, molar mass and formula mass are both based on C12 scale. However, the unit for molar mass is (gmol-1), and for formula mass is (amu). Can someone please explain what is the difference that causing people using two different units?

sjb

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Re: A conceptual problem about mass of atoms
« Reply #1 on: January 29, 2015, 03:03:44 AM »
As I know, molar mass and formula mass are both based on C12 scale. However, the unit for molar mass is (gmol-1), and for formula mass is (amu). Can someone please explain what is the difference that causing people using two different units?

Basically, the mole is an overgrown dozen. Because atoms are too small to count individually, it was decided that a certain number of atoms is a mole (currently the same as the number of atoms in 12.000... grams of 12C.

Borek

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Re: A conceptual problem about mass of atoms
« Reply #2 on: January 29, 2015, 03:27:35 AM »
As I know, molar mass and formula mass are both based on C12 scale.

I don't think that's a good approach. It is probably better to think in terms of defining the amu in such a way C-12 scale works the way it does.
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mjc123

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Re: A conceptual problem about mass of atoms
« Reply #3 on: January 29, 2015, 06:06:40 AM »
They refer to different things, although because of the way they are defined, they have the same numerical value. Thus one molecule of water has a mass of (in round figures) 18 amu, that is 18/12 times the mass of a 12C atom. One mole of water has a mass of 18 g. Hence the different units.