(a) Atomic mass of Oxygen = 16 u

Molecular mass of Oxygen (O₂) = 16 x 2 = 32 g

Now, 1 mole of oxygen gas = Molecular mass of oxygen in grams = 32 g

Also, 32g of oxygen gas = 1 mole

12 g of Oxygen gas = 1/32 X 12 = 0.375 mole

Hence, 12 g of Oxygen gas = 0.375 mole

(b) Molecular mass of water (H₂0) = 1 x 2 + 16 = 18u.

Now, 1 mole of water = Molecular mass of water in grams = 18 g

If 18 g of water = 1 mole

Then 20 g of water = 1/18 X 20 mole = 1.12 moles

Hence, 20 g of water = 1.12 moles

(c) Molecular mass of carbon dioxide (CO₂) = 12 + 16 x 2 = 12 + 32 = 44 u.

Now, 1 mole of carbon dioxide = Molecular mass of carbon dioxide in grams = 44g

If 44g of carbon dioxide = 1 mole

Then, 22 g of carbon dioxide =22/44 = 0.5 mole

Hence, 22 g of carbon dioxide = 0.5 mole