Hard water often contains dissolved [Mg]²⁺ ions. Some laundry detergents contain sodium phosphate to soften hard water and to control acidity, thus making the detergent more effective.
What solid product forms when aqueous solutions of sodium phosphate and magnesium chloride are combined?

• A. NaCl
• B. Mg₃[PO₄]₂
• C. MgCl₂
• D. Na₃PO₄
• E. Two solids form in this reaction.
• F. No solid forms in this reaction.

Answer: (B)

Verified step by step guidance

1. Identify the ions present in the solutions: Sodium phosphate (Na3PO4) dissociates into 3 Na+ ions and 1 PO4^3- ion. Magnesium chloride (MgCl2) dissociates into 1 Mg^2+ ion and 2 Cl- ions.
2. Determine the possible combinations of cations and anions that could form a precipitate. The potential combinations are Na+ with Cl-, Na+ with PO4^3-, Mg^2+ with Cl-, and Mg^2+ with PO4^3-.
3. Use solubility rules to determine which combinations form insoluble compounds. According to solubility rules, most phosphates are insoluble except those of alkali metals and ammonium. Therefore, Mg3(PO4)2 is likely to be insoluble.
4. Write the balanced chemical equation for the formation of the precipitate: 3 Mg^2+ (aq) + 2 PO4^3- (aq) → Mg3(PO4)2 (s).
5. Conclude that the solid product formed when sodium phosphate and magnesium chloride solutions are combined is magnesium phosphate, Mg3(PO4)2.