What is the mass of:
(a) 0.2 mole of oxygen atom?

(b) 0.5 mole of water molecules?

(a) Mass of 1 mole of oxygen atoms = 16u

\(\therefore \) Mass of 0.2 moles of oxygen atoms = 0.2 × 16 = 3.2u

(b) Mass of 1 mole of water molecules = 18u

\(\therefore \) Mass of 0.5 moles of water molecules = 0.5 × 18 = 9u