A reagent that will precipitate the negative ion in each dissolved compound?

Question

For MgCl2

and Na2SO4

Answer 1

I agree with chemmie but I would choose BaCl2. Ba(OH)2 is somewhat insoluble.

Answer 2

Check your solubility rules and pick things that are insoluble with Cl- and SO4^2-, such as Ag+ for your chloride ion and Ba2+ for your sulfate ion. Lots of choices there. For an actual reagent in those cases, just pick a soluble compound with those cations, such as AgNO3 and Ba(OH)2.

Answer 3

Add Silver Nitrate. The Silver ion will react with the Cl- and sulfate ions to form AgCl and Ag2SO4 precipitates.