Answer
(a) $CaSO_4$ or $SrSO_4$ or $BaSO_4$ or $PbSO_4$ or $Ag_2SO_4$.
(b) $Ni(OH)_2$ or $NiCO_3$ or $NiS$.
(c) $AgBr$ or $Hg_2Br_2$ or $PbBr_2$
Work Step by Step
(a) According to the figure 3.10 (Section 3-5), the sulfate salts with $Ca^{2+}$, $Sr^{2+}$, $Ba^{2+}$, $Pb^{2+}$ or $Ag^+$ are insoluble.
(b) According to the figure 3.10, almost all hydroxides, sulfide and carbonate are insoluble. Nickel ones are not exceptions.
(c) According to the figue 3.10 (Section 3-5), almost all salts of $Br^-$ are soluble, with the exception of the ones with $Ag^+$, $Pb^{2+}$ or $Hg_2^{2+}$.
