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So, I was just pretty confused about how we simply multiply the "mass" of total nucleons in Carbon by the Avogadro Constant, to get the molar "weight" as 12 grams, shouldn't it be the molar "mass", as we have not considered the effect of g anywhere ?

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  • $\begingroup$ Technically, it is mass. But, since the calculations aren't changed by using weight instead of mass (g, the acceleration due to gravity, is just a constant multiplier), "molecular weight" and "molecular mass" are often used interchangeably, where it's understood that "weight" means "weight under earth gravity". But, yes, using "mass" would be more technically precise. Essentially, it's no different from laboratory scales expressing their weight readouts as a mass (typically grams), when what they are really measuring is a force, which should instead be expressed in newtons. $\endgroup$
    – theorist
    Jun 24 '21 at 5:01
  • $\begingroup$ 2 g hydrogen atoms (H): $\frac{2 g}{1 g/mol}= 2\, mol$. 2 g hydrogen gas ($\ce{H_2(g)}$): $\frac{2 g}{2 g/mol}= 1 \,mol$. $\endgroup$
    – theorist
    Jun 24 '21 at 5:43
  • $\begingroup$ You are confused by the usage of the words, mass and weight. Try searching for their definitions. $\endgroup$ Jun 25 '21 at 3:17
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This is a linguistic issue rather than physics or chemistry. In older literature of chemistry texts you will find that atomic weights was more frequently used, so the terms atomic mass and weights were interchangeably used by various authors. Recall that an everyday luxury called electricity was not available even to scientists 100 or 300 years ago. "Masses" of objects were determined by sensitive mechanical balances like these until the 1960-70s. On one side you had the object whose "mass" you were interested in and on the other side, a known weight was placed. You were basically comparing the weights (mass x acceleration due to gravity "g"), however, since "g" is same on both arms, it is mathematically cancelled, and we get mass of the object.

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The International Union of Pure and Applied Chemistry has a committee called "Commission on Isotopic Abundances and Atomic Weights." See IUPAC's atomic weights

Another minor correction in your post is that 2+2 is not equal to 4 at sub-atomic levels. The mass of 1 mol of C-12 atoms (not the sum of nucleons) is assigned by humans as 12 grams exactly. Recent revisions have slightly changed this value, but keep in mind that the sum of the masses of 1 mole of total nucleons of C-12 atoms does not sum up to 12.

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  • $\begingroup$ Oh ok sir, but I still have some unanswered question. If I go by this explanation, I can use the term mass and weight interchangeably because they will "cancel out". So, I calculated say, that 1 mole of carbon dioxide should weigh 44 grams. I go to my laboratory and measure the gas, and come up with 440 grams for a certain amount of the gas, and hence write that I have 10 moles of the gas, or 10 times avogadro number of molecules. But - the reality is that "by mass" I still have just 1 mole and only avogadro times molecules ! $\endgroup$ Jun 24 '21 at 12:06
  • $\begingroup$ Please clarify the point which is still not clear by adding a new edit in your original post. $\endgroup$
    – M. Farooq
    Jun 24 '21 at 12:09
  • $\begingroup$ If I go by this explanation, I can use the term mass and weight interchangeably because they will "cancel out". So, I calculated say, that 1 mole of carbon dioxide should weigh 44 grams. I go to my laboratory and measure the gas, and come up with 440 grams for a certain amount of the gas, and hence write that I have 10 moles of the gas, or 10 times avogadro number of molecules. But - the reality is that "by mass" I still have just 1 mole and only avogadro times molecules ! @M.Farooq $\endgroup$ Jun 24 '21 at 12:45
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    $\begingroup$ Abigail, Yes, mass and weight are generally used interchangeably but mass is what you get on balances not weight = mg. There is no role of "g" in the numbers you read on a balance. $\endgroup$
    – M. Farooq
    Jun 24 '21 at 13:23

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