Why is Amount of Mole in Grams Equal to Atomic Mass?

[batballbat]

Why is the amount of mole in gms. of an element the same as the its atomic mass. Is it just because $N_a X 1 u = 1$. Am i missing some simple reasoning behind this? Or was the carbon 12 taken exactly to make this valid?

 

[Borek]

Simple answer - atomic unit was selected to make it happen.

N_a X 1 u = 1

Or was the carbon 12 taken exactly to make this valid?

In a way you listed the same reason twice. Note what happens when you list units:

N_a[\frac{atom}{mole}] \times u[\frac{g}{atom}] = 1[\frac{g}{mole}]

 

[morrobay]

You can also take a numerical example :

1 atomic mass unit = 1.6605655 x 10^-24 grams
Once it is determined that 1 gram Hydrogen contains :
1 g. H / 1.6605655 x 10^-24 g = 6.02 x 10²³
1 mole H = Avogadro's number. Then it follows for 1 mole of any other element.
Take 40 grams of Zr :
40 g. Zr/ 40 * 1.660565 x 10^-24 g. =
40 g. Zr / 6.642 x 10^-23 g = 6.02 x 10 ²³

 

[morrobay]

You can also take a numerical example :

1 atomic mass unit = 1.6605655 x 10^-24 grams
Once it is determined that 1 gram Hydrogen contains :
1 g. H / 1.6605655 x 10^-24 g = 6.02 x 10²³
1 mole H = Avogadro's number. Then it follows for 1 mole of any other element.
Take 40 grams of Zr :
40 g. Zr/ 40 * 1.660565 x 10^-24 g. =
40 g. Zr / 6.642 x 10^-23 g = 6.02 x 10 ²³

This should be 91 grams Zr :
so : 91 grams Zr / 91 * 1.66 x 10 ^ -24 g
= 91 g Zr / 15.1 x 10^-23 g = 6.02 x 10^23