Write the molecular formulae for the following compounds:
$(a)$ Copper $(II)$ bromide
$(b)$ Aluminium $(III)$ nitrate
$(c)$ Calcium $(II)$ phosphate
$(d)$ Iron $(III)$ sulphide
$(e)$ Mercury $(II)$ chloride
$(f)$ Magnesium $(II)$ acetate

(N/A) Copper $(II)$ bromide: The valency of $Cu$ is $2$ and $Br$ is $1$. Thus,the formula is $CuBr_2$.
$(b)$ Aluminium $(III)$ nitrate: The valency of $Al$ is $3$ and $NO_3$ is $1$. Thus,the formula is $Al(NO_3)_3$.
$(c)$ Calcium $(II)$ phosphate: The valency of $Ca$ is $2$ and $PO_4$ is $3$. Thus,the formula is $Ca_3(PO_4)_2$.
$(d)$ Iron $(III)$ sulphide: The valency of $Fe$ is $3$ and $S$ is $2$. Thus,the formula is $Fe_2S_3$.
$(e)$ Mercury $(II)$ chloride: The valency of $Hg$ is $2$ and $Cl$ is $1$. Thus,the formula is $HgCl_2$.
$(f)$ Magnesium $(II)$ acetate: The valency of $Mg$ is $2$ and $CH_3COO$ is $1$. Thus,the formula is $Mg(CH_3COO)_2$.