Question

8g of oxygen contains_ number of atoms

Answer 1

First calculate ther number of moles of O2 using: moles = mass / relative molecular mass = 8 / 32 = 0.25. 
For each molecule of O2, there are 2 oxygen atoms. Therefore moles of oxygen atoms = 2 x 0.25 = 0.5 

1 mole of anything is 6.022 x 10^23 atoms (Avogadro constant) 
So number of atoms = 0.5 x 6.022 x 10^23 = 3.011 x 10^23 

Answer 2

mass of o=16 g=1 mole=6.022*10²³
8g oxygen= 1/2 mole
1/2 mole= 3.011*10²³ atoms