Which of the following phosphate salts is insoluble in water?

• A. K3PO4
• B. Na3PO4
• C. NH4PO4
• D. Ca3(PO4)2

Answer: (D)

Verified step by step guidance

1. Recall the general solubility rules for ionic compounds in water, especially for phosphates (PO4^3-). Most phosphates are insoluble except those formed with alkali metal ions (such as K^+, Na^+) and the ammonium ion (NH4^+).
2. Identify the cations in each given phosphate salt: K3PO4 contains potassium (K^+), Na3PO4 contains sodium (Na^+), NH4PO4 contains ammonium (NH4^+), and Ca3(PO4)2 contains calcium (Ca^{2+}).
3. Apply the solubility rules: phosphates of alkali metals (K^+, Na^+) and ammonium (NH4^+) are generally soluble in water, so K3PO4, Na3PO4, and NH4PO4 should dissolve.
4. Recognize that calcium phosphate, Ca3(PO4)2, is an exception and is known to be insoluble in water due to the low solubility of most alkaline earth metal phosphates.
5. Conclude that among the given options, Ca3(PO4)2 is the phosphate salt that is insoluble in water.