Which of the following salts is LEAST soluble in water? — Solutions Chemistry Question

STEPS:

1. Identify the Core Concept: This question tests the application of solubility rules for common ionic compounds in aqueous solutions.
2. Evaluate Group 1 and Nitrate Salts: Recall the most fundamental solubility rules: all salts containing alkali metal cations (like $K^+$) or the nitrate anion ($NO_3^-$) are highly soluble in water. This immediately classifies Choice C ($K_2CrO_4$) and Choice E ($Pb(NO_3)_2$) as soluble.
3. Evaluate Halide and Sulfate Solubility: Most chlorides ($Cl^-$) and sulfates ($SO_4^{2-}$) are generally soluble. Common exceptions for halides include $Ag^+$, $Pb^{2+}$, and $Hg_2^{2+}$; for sulfates, exceptions include $Ba^{2+}$, $Sr^{2+}$, and $Pb^{2+}$. Since neither $Mg^{2+}$ nor $Al^{3+}$ are among these exceptions, Choice B ($MgCl_2$) and Choice D ($Al_2(SO_4)_3$) are also soluble.
4. Evaluate Sulfide Solubility: Recall the rule for sulfides ($S^{2-}$): most transition metal sulfides are insoluble in water (having very low $K_{sp}$ values) unless they are paired with Group 1 or Group 2 cations.
5. Conclusion: Because NiS (nickel(II) sulfide) is a transition metal sulfide and does not fall under any "soluble" category, it is the LEAST soluble salt among the provided options.

WHY_OTHERS_WRONG:

• $MgCl_2$ (B): This is a typical soluble halide; magnesium chloride dissociates completely in water.
• $K_2CrO_4$ (C): The presence of the potassium ion ($K^+$) guarantees high solubility in water regardless of the anion.
• $Al_2(SO_4)_3$ (D): Aluminum sulfate is a common soluble salt used in water treatment; it does not meet the criteria for insoluble sulfate exceptions.
• $Pb(NO_3)_2$ (E): While lead(II) often forms precipitates with other anions (like halides or sulfates), all nitrate salts are soluble without exception.