What is the mass of 0.2 mole of oxygen atoms?

(a) The mass of 1 mole of oxygen atoms = 16 g
Thus, the mass of 0.2 mole of oxygen atoms = 16 × 0.2 = 3.2 g
(b) The mass of 1 mole of water molecules = 18 g
Thus, the mass of 0.5 mole of water molecules = 18 × 0.5 = 9.0 g