A clear, colorless unknown solution was divided into four separate test tubes. Nothing was observed when KOH was added to the first test tube. Nothing was observed when AgNO3(aq) was added to the third. A white precipitate formed when BaCl2(aq) was added to the fourth test tube. Which of the possible unknowns for this experiment would give this behavior? Write a balanced equation for the reaction observed between this compound and BaCl2.