The compound insoluble in water is

According to "solubility rule" following compounds are soluble in water: (i) All common group IA and $\mathrm{NH}_4^{+}$compounds. (ii) $\mathrm{All}$ common $\mathrm{NO}_3^{-}$(nitrate), $\mathrm{CH}_3 \mathrm{COO}^{-}$(acetate), $\mathrm{ClO}_4^{-}$(perchlorate) compounds (iii) $\mathrm{All}$ common $\mathrm{Cl}^{-}, \mathrm{Br}^{-}, \mathrm{I}^{-}$compounds, except those of $\mathrm{Ag}^{+}, \mathrm{Pb}^{2+}, \mathrm{Cu}^{+}$and $\mathrm{Hg}_2^{2+}$ (mercurous) (iv) All common $\mathrm{SO}_4{ }^{2-}$ (sulphate) compounds, except those of $\mathrm{Ca}^{2+}, \mathrm{Sr}^{2+}, \mathrm{Ba}^{2+}$ and $\mathrm{Pb}^{2+}$.