Question

what is th mass in gram of oxygen atom and molecule

Answer 1

One mole is a quantity of $\mathrm{N}_0$ atoms or molecules. Therefore, the atomic weight is the weight of one mole of atoms, and the molecular weight is the weight of one mole of molecules. The $\mathrm{N}_0$ is Avogadro's number, and it's equal to the following:

$$\mathrm{N}_0 = 6.022 \cdot 10^{23}$$

The molecular weight Mw of a molecule, multiplied by the number of moles n, is equal to the total weight W of the molecules:

$$\mathrm{W} = \mathrm{n} \cdot \mathrm{Mw}$$

So one mole includes $6.022 \cdot 10^{23}$ particles.

For $\mathrm{O}_2$, one mole has weight of $32\,\mathrm{g}$ (from molecular weight)

mass of one molecule $= 6.022 \cdot 10^{23} / 32\,\mathrm{g} = 5.314 * 10^{-23}\,\mathrm{g}$

mass of one atom is $5.314 * 10^{-23}\,\mathrm{g} / 2 = 2,657 * 10^{-23}\,\mathrm{g}$