calculate -

a)no. of grams-atoms of oxygen ?

b)the no. of atoms of oxygen

c)the no. of molecules of ozone in 32g of ozone (O3)

To calculate the number of atoms you need certain amount of samples.
Let us say calculate the no. of atoms in 8 gram oxygen.
Formula used : number of moles of atom = given mass /atomic mass of atom
Number of moles of Oxygen atom in 8 gram sample = 8g / 16g = 0.5 moles
1 mole = 6.023 x 10²³ atoms
0.5 mole = 0.5 x 6.023 x 10²³ atoms = 3. 01 x 10²³ atoms

iii) Atomic mass of ozone = 48 gram
Number of moles of ozone in 32 gram sample = 32 g / 48g = 0.67 mole
1 mole = 6.023 x 1023 atoms/molecules
Number of molecules of ozone in given sample = 0.67 x 6.023 x 10²³ molecules = 4.01 x 10²³ molecules.