Which of the following compounds is the least soluble in water?

Analyze the question: We need the compound with the lowest solubility in water. This depends on the solubility rules for salts.

Concept: Solubility rules

• All sodium (Na⁺) salts are soluble.
• All chlorides are soluble, except AgCl, PbCl₂ and Hg₂Cl₂.
• Most sulfates are soluble, except BaSO₄, PbSO₄ and CaSO₄ (CaSO₄ is only sparingly soluble).

Step-by-step solution:

• A. CaCl₂: a chloride, not one of the exceptions, so it is very soluble. In fact it is deliquescent.
• B. CaSO₄: a sulfate of calcium, one of the exceptions, so it is only sparingly soluble (about 0.2 g per 100 g of water at room temperature).
• C. NaCl: a sodium salt and a chloride, so it is highly soluble (about 36 g per 100 g of water).
• D. Na₂SO₄: a sodium salt, so it is highly soluble.

Three of the four compounds dissolve readily. Only CaSO₄ is sparingly soluble, so it is the least soluble.

Answer: B. CaSO₄

Tip: Remember the "sulfate exceptions" as Ba-Pb-Ca (BaSO₄, PbSO₄, CaSO₄). Whenever a WASSCE question asks for the least soluble salt and a sulfate of one of these metals is among the options, check it first. Also remember that all Na⁺, K⁺ and NH₄⁺ salts are soluble, so you can eliminate them quickly.