Question

Which ion combines with Ba2+ to form a compound that is most soluble in water?
(1) S2- (2) OH- (3) CO32- (4) SO42-

Answer 1

Only Ba(OH)2 is soluble in water. BaS, BaCO3, and BaSO4 are precipitates.
(2) OH- is the correct answer.
Hope this would help~

Answer 2

The chemical equation representing the reaction of $Ba^{2+}$ with each of the given ions is:

(1) $Ba^{2+}(aq)+S^{2-}(aq)-->BaS(s)$

(2)$Ba^{2+}(aq)+2OH^{-}-->Ba(OH)_{2}(aq)$

(3)$Ba^{2+}(aq)+CO_{3}^{2-}(aq)-->BaCO_{3}(s)$

(4)$Ba^{2+}(aq)+SO_{4}^{2-}(aq)-->BaSO_{4}(s)$

Sulfates, carbonates, phosphates, sulfides, and chromates of barium are insoluble. whereas acetates, nitrates, hydroxides, and chlorides of barium are soluble.

Therefore, among the compounds formed from the given ions barium sulfide, barium carbonate and barium sulfate are insoluble.

Hence, the correct options are (1) $S^{2-}$ (3) $CO_{3}^{2-}$ (4) $SO_{4}^{2-}$