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Why is that molar mass has of a substance, which is the mass of one mole of a substance ($6.02\times 10^{23}$ particles of that substance) has the same value as molecular mass, which is the mass of one molecule which in some cases is just one of the particles stated above (one particle from a mole). Also why do they have different units?

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    $\begingroup$ Say, a coin weighs 2 g. Then a thousand coins weighs 2 kg. How come these values (2 and 2) are similar, and why are they in different units? $\endgroup$ – Ivan Neretin Feb 17 '19 at 14:53
  • $\begingroup$ Thank you that was surprisingly the most useful answer I have gotten throughout but are you sure it is correct $\endgroup$ – Taofeek Feb 17 '19 at 15:55
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    $\begingroup$ An answer? I never gave you any. I merely asked a rhetorical question; if it made you think in the right direction, good for you. Just what part of my comment could be mistaken for something that may or may not be correct? That a certain real-world coin actually weighs 2 g? I never said that. Or that 1000 times 2 g makes 2 kg? Well, yes, I guess I can say so with a good deal of confidence. $\endgroup$ – Ivan Neretin Feb 17 '19 at 16:08
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    $\begingroup$ Related: Quick and simple explanation of molar mass, molecular mass and atomic mass $\endgroup$ – Faded Giant Feb 17 '19 at 19:25
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molar mass of a substance is the mass of one mole of a substance

This is not quite correct. If you said the speed of a car is the distance it travels in one hour, cars would go at speeds of 120 miles (race car) or 30 miles (in town). However, speed is defined as distance traveled per time. As a result, speed has the dimension of distance divided by time, and a typical unit (at least where I live) is miles per hour.

Molar mass is defined as the mass per amount of substance. The dimensions are mass divided by amount of substance, and typical units are g/mol.

Why does molar mass have the same value as molecular mass?

If the molar mass is given in g/mol and the molecular mass is given in u (unified atomic mass unit) or Dalton, the number is the same. The reason is that both the mole (at least until May 2019) and the Dalton are based on the mass of the carbon-12 isotope. A mole of a substance is defined as the amount that has the same number of particles as present in 12 grams of carbon-12. The Dalton is define as 1/12th of the mass of a carbon-12 atom.

From these definitions, you will find that the molar mass of the carbon-12 isotope is 12 g/mol, exactly, and the molecular mass of carbon-12 is 12 Dalton, exactly.

What will change in May 2019?

The mole will be defined as "One mole contains exactly 6.02214076×10^23 elementary entities." The molar mass of carbon-12 will be an experimentally determined quantity.

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  • $\begingroup$ I'd change the sentence "If the molar mass is given in g/mol and the molecular mass is given in u..." to If the molar mass is given in g/mol and the molecular mass is given in u/atom ..." $\endgroup$ – MaxW Feb 17 '19 at 18:48
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    $\begingroup$ I would argue that molar mass by definition is the mass of 1 mole of substance. Otherwise you can call it something else. The term "molar" speaks for itself, just as in other molar quantities. $\endgroup$ – Buck Thorn Feb 17 '19 at 19:31
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    $\begingroup$ @Try_Hard : As long as it is clear that the dimensions are mass divided by amount of substance, either definition is fine. I'm not a native English speaker, so to me the mass of 1 mole of substance sounds like the dimensions would be mass, while the mass per amount of substance (or the mass divided by the amount of substance) sounds like the dimensions are mass divided by amount of substance. Some definitions just add "expressed in g/mol" to make that clear, which is fine too. $\endgroup$ – Karsten Theis Feb 17 '19 at 22:41
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    $\begingroup$ @MaxW : I think it is fine to say in the text that this refers to the mass of one atom (or call the quantity $m_\mathrm{atom}$), but I'm not so sure about putting it into the units. What is the dimension of "atom"? What is the value of "atom". If it is 1, the statements are still true. The definition of molecular mass already includes that it refers to a single molecule (or atom). $\endgroup$ – Karsten Theis Feb 17 '19 at 22:46
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    $\begingroup$ Kirsten basically the Avogadro number will be taken as exactly that. So we shall expect in principle little change in the atomic mass for atoms different than C 12. It is a shame but I cannot quickly figure it out $\endgroup$ – Alchimista Feb 18 '19 at 10:18

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