$(a)$ What do the following formulae stand for:
$(i)$ $2O$
$(ii)$ $O_{2}$
$(iii)$ $O_{3}$
$(iv)$ $H_{2}O$
$(b)$ Give the chemical formulae of the following compounds:
$(i)$ Potassium carbonate
$(ii)$ Calcium chloride
$(c)$ Calculate the formula unit mass of $Al_{2}(SO_{4})_{3}$.
(Given atomic mass of $Al = 27 \, u, S = 32 \, u, O = 16 \, u$)

(N/A) $(i)$ $2O$ represents two separate atoms of oxygen.
$(ii)$ $O_{2}$ represents one molecule of diatomic oxygen.
$(iii)$ $O_{3}$ represents one molecule of triatomic oxygen (ozone).
$(iv)$ $H_{2}O$ represents one molecule of water, consisting of two atoms of hydrogen and one atom of oxygen.
$(b)$ $(i)$ Potassium carbonate: $K_{2}CO_{3}$
$(ii)$ Calcium chloride: $CaCl_{2}$
$(c)$ Formula unit mass of $Al_{2}(SO_{4})_{3}$:
$= (2 \times \text{atomic mass of } Al) + (3 \times \text{atomic mass of } S) + (12 \times \text{atomic mass of } O)$
$= (2 \times 27) + (3 \times 32) + (12 \times 16)$
$= 54 + 96 + 192 = 342 \, u$.