Who proposed the laws of chemical combination?

$(a)$ 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 $Al = 27 \, u, S = 32 \, u, O = 16 \, u$)

Find the total number of atoms present in one $SO_{4}^{2-}$ ion.

$A$ change in the physical state can be brought about:

Fill in the missing data in the Table.

Species property	$H_2O$	$CO_2$	$Na$ atom	$MgCl_2$
No. of moles	$2$	-	-	$0.5$
No. of particles	-	$3.011 \times 10^{23}$	-	-
Mass	$36 \, g$	-	$115 \, g$	-

The chemical symbol for nitrogen gas is