What is a molecular formula? State with an example what information can be derived from a molecular formula.

(N/A) molecular formula represents the actual number of atoms of each element present in one molecule of a compound.
For example,the molecular formula of aluminium sulphate is $Al_{2}(SO_{4})_{3}$. It provides the following information:
$(i)$ The compound is composed of three different types of atoms: aluminium $(Al)$,sulphur $(S)$,and oxygen $(O)$.
$(ii)$ There are a total of $17$ atoms in one molecule ($2$ atoms of $Al$,$3$ atoms of $S$,and $12$ atoms of $O$).
$(iii)$ The valency of aluminium $(Al^{3+})$ is $3$,while the valency of the sulphate ion $(SO_{4}^{2-})$ is $2$.