Question #c6de3

This problem wants to test your knowledge of which ionic compounds are soluble and which are insoluble in aqueous solution.

In order to be able to answer this question, you must be familiar with the solubility rules for ionic compounds in aqueous solution

So, you know that your solution is treated with potassium chloride, #"KCl"#, sodium sulfate, #"Na"_2"SO"_4#, and sodium hydroxide, #"NaOH"#.

All of these ionic compounds are soluble in aqueous solution, which means that they exist as cations and anions when dissolved in water

#"KCl"_text((aq]) -> "K"_text((aq])^(+) + "Cl"_text((aq])^(-)#

#"Na"_2"SO"_text(4(aq]) -> 2"Na"_text((aq])^(+) + "SO"_text(4(aq])^(2-)#

#"NaOH"_text((aq]) -> "Na"_text((aq])^(+) + "OH"_text((aq])^(-)#

You're looking for cations, which are positively charged ions, that will combine with the sulfate anions, #"SO"_4^(2-)#, to form an insoluble solid that precipitates out of solution.

Moreover, you need these cations to not form an insoluble solid with the chloride anions, #"Cl"^(-)#, or with the hydroxide anions, #"OH"^(-)#.

As you can see in the list, the only cation that is of interest here is the barium cation, #"Ba"^(+)#.

Barium cations will not form an insoluble solid with the chloride anions, since barium chloride, #"BaCl"_2#, is itself a soluble ionic compound.

Likewise, barium hydroxide, #"Ba"("OH")_2#, is also soluble in aqueous solution, so the barium cations will not react with the hydroxide anions to form this ionic compound.

However, barium cation will react with the sulfate anions to form barium sulfate, #"BaSO"_4#, a white insoluble solid that will precipitate out of solution.

Therefore, your unknown soluble ionic compound most likely contains barium cations, #"Ba"^(2+)#.