An acid (X) reacts with a base (Y) to produce Mg3(PO4)2. What are X and Y?

The product Mg3(PO4)2 contains magnesium ions and phosphate ions. Magnesium hydroxide, Mg(OH)2, supplies magnesium, while phosphoric acid, H3PO4, supplies phosphate. They react as 2H3PO4 + 3Mg(OH)2 → Mg3(PO4)2 + 6H2O, so the correct choice is B: X = H3PO4 and Y = Mg(OH)2.

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