Answer
Barium Chloride
Work Step by Step
When you first place the solid in water, it dissolves, meaning that you have Cl- ions and either Barium, lead or zinc ions. Then, when you mix the solution with the Na2SO4, you will get a reaction that will form NaCl, which is soluble and then it will also form either BaSO4, PbSO4 or ZnSO4. Since a white precipitate forms, we know that this compound must be water insoluble and of the three, we know that Barium Sulfate is not soluble in water. Therefore, we must have started with BaCl2
