Separate samples of a solution of an unknown soluble ionic compound are treated with KCl, Na2SO4, and NaOH. A precipitate forms only when Na2SO4 is added. Which cations could be present in the unknown soluble ionic compound?

Ba⁺², Sr⁺², Ca⁺², and Ra⁺²

Explanation:

If a precipitate is formed, then the ionic compound (a salt or a base) will not be soluble in water. That compound is formed by a metal cation and the anions of the substances given: Cl⁻, SO₄⁻² and OH⁻. With Cl⁻ and SO₄⁻² it will form a salt, and with OH⁻ it will form a base.

The insoluble substance is formed only with SO₄⁻², and most compounds formed with this anion are soluble, the exceptions are the cations Ba⁺², Sr⁺², Ca⁺², Ra⁺², and Pb⁺². The salt formed between Pb⁺² and Cl⁻ is also insoluble, so, Pb⁺² can't be one of the cations.

The soluble base is the ones formed by metals from the groups I and II, all the others are insoluble and would form a precipitated. Ba, Sr, Ca and Ra are from group 2, so they form soluble bases.

Then, the possible cations are: Ba⁺², Sr⁺², Ca⁺², and Ra⁺².