(a) The atomic mass of oxygen (O) = 16 u.

The mass of 1 mole of oxygen atoms = 16g.

Now, 1 mole of oxygen atoms = 16g

So, 0.2 mole of oxygen atoms = 16 x 0.2g = 3.2g

Hence, 0.2 mole of oxygen atom = 3.2 g.

(b) The molecular mass of water (H₂0)= 18u,

The mass of 1 mole of water molecules = 18 g

Now, 1 mole of water molecules = 18g

0.5 mole of water molecules = 18 x 0.5g = 9g

Hence, 0.5 mole of water molecules = 9 g