Write the formulae for the following compounds:
$(a)$ Mercury $(II)$ chloride
$(b)$ Nickel $(II)$ sulphate
$(c)$ Tin $(IV)$ oxide
$(d)$ Thallium $(I)$ sulphate
$(e)$ Iron $(III)$ sulphate
$(f)$ Chromium $(III)$ oxide

(N/A) Mercury $(II)$ chloride: $Hg^{2+}$ and $Cl^-$ ions combine to form $HgCl_2$.
$(b)$ Nickel $(II)$ sulphate: $Ni^{2+}$ and $SO_4^{2-}$ ions combine to form $NiSO_4$.
$(c)$ Tin $(IV)$ oxide: $Sn^{4+}$ and $O^{2-}$ ions combine to form $SnO_2$.
$(d)$ Thallium $(I)$ sulphate: $Tl^+$ and $SO_4^{2-}$ ions combine to form $Tl_2SO_4$.
$(e)$ Iron $(III)$ sulphate: $Fe^{3+}$ and $SO_4^{2-}$ ions combine to form $Fe_2(SO_4)_3$.
$(f)$ Chromium $(III)$ oxide: $Cr^{3+}$ and $O^{2-}$ ions combine to form $Cr_2O_3$.