Which one of the following compounds would form a precipitate when mixed with aqueous Ba(NO3)2?

• A. NH4NO3
• B. LiBr
• C. Na2SO4
• D. KCl

Answer: (C)

Verified step by step guidance

1. Identify the ions present in aqueous Ba(NO3)2. It dissociates into Ba^{2+} and NO_3^{-} ions in solution.
2. Consider each compound and determine the ions they produce when dissolved in water: NH_4NO_3 gives NH_4^{+} and NO_3^{-}, LiBr gives Li^{+} and Br^{-}, Na_2SO_4 gives Na^{+} and SO_4^{2-}, and KCl gives K^{+} and Cl^{-}.
3. Recall the solubility rules, especially that most sulfate salts are soluble except those with Ba^{2+}, Pb^{2+}, Hg_2^{2+}, and Ca^{2+}, which form precipitates.
4. When Ba^{2+} ions from Ba(NO_3)_2 meet SO_4^{2-} ions from Na_2SO_4, they form BaSO_4, which is insoluble and precipitates out of solution.
5. For the other compounds, their ions do not form insoluble salts with Ba^{2+}, so no precipitate forms when mixed with Ba(NO_3)_2.