Moles and Avogadro's number

A mole is defined as the number of atoms present in 12 grams of ¹²C. Since the atomic mass of ¹²C is 12 amu, and 1 amu is 1.66x10^-24 g1.66 times ten to -24 grams, then 12 grams of ¹²C must contain 6.022x10²³ 6.022 times ten to 23atoms:

The equation showing the transition from one mole of carbon to the Avogadro number. 12 grams per 12 amu equals 1 gram per 1 amu equals 1 gram per 1.66 times 10 to the power of minus 24 grams, which equals 6.022 times 10 to the power of 23.

This number is also referred to as Avogadro’s number in recognition of the great Italian chemist Amedeo Avogadro.

It follows from this that since the mass of one atom of ¹²C is 12 amu, then the mass of one mole of ¹²C is 12 grams. The latter is called the molar mass and its unit are grams per mole (g/mol or gᐧmol^-1). This is true for every atom in the periodic table: The mass stated in the periodic table is the mass of one atom in amu, but it is also the mass of one mole of those atoms in grams (molar mass).

Examples: The atomic mass of Iron (Fe) is 55.845. It means that one atom of iron weighs 55.845 amu and that one mole of iron weighs 55.845 g. The molecular mass of water is 18.015. It means that one molecule of water weighs 18.015 amu and that one mole of water weighs 18.015 g.

You can read about how to count atoms and molecules here

Referred from:

Article

How to count molecules