Question

According to Avogadro's number, why there is one mole in 12 grams of oxygen? And why not in 8 grams or another amount of oxygen?

Answer 1

One mole of oxygen atoms contains $6.02214179\times {10}^{23}$ oxygen atoms.
The number $6.02214179\times {10}^{23}$ alone is called Avogadro's number.

Each carbon-12 atom weighs about; therefore,
$(1.99265\times {10}^{-23}g)\times (6.02214179\times {10}^{23})$ atoms = 12 g of carbon = $12(1.99265\times {10}^{-23}g)\times (6.02214179\times {10}^{23})$ atoms = 12 g of carbon - 12
$(1.99265\times {10}^{-23}g)\times (6.02214179\times {10}^{23}atoms)$ = 12g of carbon - 12