Avogadro’s Number and the Molar Mass of an Element

Atomic mass units provide a relative scale for the masses of the elements. But because the mass of atoms is so tiny it is more convenient to have a special unit to describe the number of atoms.

Like for example the pair (2 items) and the dozen (12 items), chemists measures atoms and molecules in moles. One mole of carbon-12 isotope is exactly equal to 12 grams of carbon-12. The number of atoms in one mole of carbon-12 (12 grams) is equal to 6.0221415 X 10²³ atoms, round off to 6.022X10²³ atoms. This number is called Avogadro’s number and takes the symbol (N_A).

One mole of an element or one mole of molecules has the Avogadro’s number (6.0221415 X 10²³ atoms). The 12 grams of carbon-12 is called the molar mass. Also, the molar mass of carbon-12 (in grams) is numerically equal to its atomic mass in amu. That is applied to the other element too. Atomic mass of Calcium (Ca) is 40.078 amu and molar mass is 40.078 grams.

By that, if we know the atomic mass of an element, we also know its molar mass.

If we know molar mass and Avogadro’s number, we can calculate the mass of a single atom in grams. For example, the molar mass of carbon-12 is 12.00 grams contains 6.022 X 10²³ atoms.

Therefore, the mass of one carbon-12 atom is given by dividing the grams of carbon over the number of atoms:

Mass of one carbon-12 atom = 

To find the relation between atomic mass units and grams:

 

1amu = 6.022X10²³ grams