(i) (a) 2O = 2 atoms of oxygen
(b) O2 = Diatomic oxygen
(c) O3 = Triatomic oxygen / ozone
(d) H2O=Two atoms of hydrogen and one atom of oxygen construct one molecule of water (H2O)
(ii) Potassium carbonate - K2CO3
Calcium chloride - CaCl2
(iii) Formula unit mass of any compound is the sum of atomic masses of all atoms in the formula of that compound.
Formula unit mass of Al2(SO4)3
Atomic mass of Al = 27 × 2 = 54 u
Atomic mass of S = 32 × 3 = 96 u
Atomic mass of O = 16 × 12 = 192 u
Formula unit mass = 54 + 96 + 192 = 342 u.
Hence, Formula unit mass of Al2(SO4)3 is 342.