Give the chemical formulas for the following compounds:
$(i)$ Ammonium molybdate
$(ii)$ Ammonium phosphomolybdate
$(iii)$ Ferric thiocyanate ion
$(iv)$ Sodium sulphide

(N/A) $(i)$ Ammonium molybdate is $(NH_4)_2MoO_4$.
$(ii)$ Ammonium phosphomolybdate is $(NH_4)_3[PMo_{12}O_{40}]$.
$(iii)$ Ferric thiocyanate ion is $[Fe(SCN)]^{2+}$.
$(iv)$ Sodium sulphide is $Na_2S$.