3) What is meant by the term chemical formula?
4) How many atoms are present in a
(i) $H_{2}S$ molecule and
(ii) $P{O}_4^{3-}$ ION ?

$3)$ A chemical formula is a symbolic representation of the composition of the compound it represents the relative proportions of the elements in the compound.

$4)$ $\left(i\right)$ $H_{2}S$ molecule contains $2H$ and $1S$ atom, $i.e.$ total $3$ atoms.

$\left(ii\right)$ $P{O}_4^{3-}$ ion contains $1P$ and $4O$ atoms. $i.e.$ total $5$ atoms.