How many atoms are present in one gram atomic mass of a substance ?

Q. (a) Define gram atomic mass of a substance. How much is the gram atomic mass of oxygen ?
(b) How many moles of oxygen atoms are present in one mole of the following compounds ?

(i) Al₂O₃
(ii) CO₂
(iii) Cl₂O₇
(iv) H₂SO₄
(v) Al₂(SO₄)₃

Q. How many oxygen atoms are present in 2 gram-atoms of $O$?

Q. How many gram atoms are present in $256g$ of $O_2$?

Q. How many gram atoms are present in $4000amu$ of calcium?

Q. How many gram atoms and number of atoms are present in $20g$ of calcium?