Given mass = 64 gm
Molecular mass of SO2 = 32+(16×2) = 64 gm
No. of moles = given mass/ molar mass = 64gm/ 64 gm = 1
Now, we get 64 gm of SO2 have 1 mole of SO2 molecules. Also 1 molecule of SO2 consists of 3 atoms ( 1 of Sulphur and 2 of oxygen).
So, 1 mole of SO2 = 6.023× 10^23 molecules of SO2 = 3× (6.023× 10^23) atoms = 18.069× 10^23 atoms.