Ans: 4) PbSO4
Different compounds are soluble in water to a different extent, some of them are completely insoluble in water. Solubility rules help in identifying the water soluble and insoluble compounds.
Based on the rules:
1) Salts of group IA (Li, Na, K, Rb and Cs) are soluble in water
2) Salts containing NH4+ ions are water soluble
3) All phosphates (PO4³⁻) are insoluble except those containing group IA elements and (NH4)₃PO4
4) All sulfates, SO4²⁻ salts are generally soluble with the exception of Ca2+, Sr2+, Ba2+, Hg₂²⁺, Hg2+, Pb2+ and Ag+
Therefore, among the given examples:
1) KOH contains group 1A (K) therefore soluble
2) Na3PO4 contains group IA element (Na), therefore soluble
3) NH4Cl contains NH4+ ion, therefore soluble
4) Sulfate of lead (Pb) is insoluble
