Is the ionic compound (NH4)3PO4 soluble or insoluble, and does each of its ions tend to be soluble?
a. Both ions soluble
b. Both ions insoluble
c. Ions differ; soluble
d. Ions differ; insoluble
Correct Answer: a. Both ions soluble
Explanation:
The compound $(NH_4)_3PO_4$, known as ammonium phosphate, is composed of two types of ions: the ammonium ion (NH₄⁺) and the phosphate ion (PO₄³⁻).
To determine the solubility of this ionic compound, we refer to general solubility rules for ionic compounds in water:
1. Ammonium ion (NH₄⁺) — Always Soluble:
According to solubility rules, all compounds containing the ammonium ion (NH₄⁺) are soluble in water, regardless of the anion it is paired with. This is because the ammonium ion mimics alkali metal behavior in terms of solubility.
2. Phosphate ion (PO₄³⁻) — Generally Insoluble:
Phosphate salts are generally insoluble in water except when combined with alkali metals (e.g., Na⁺, K⁺, Li⁺) or ammonium (NH₄⁺). Thus, phosphate is not inherently soluble on its own, but it becomes soluble when paired with specific cations like NH₄⁺.
Compound Solubility:
In the case of (NH₄)₃PO₄, we have:
- Cation (NH₄⁺): Soluble by rule.
- Anion (PO₄³⁻): Soluble only when paired with NH₄⁺ or alkali metals.
Since both ions in this compound result in a soluble compound when combined, (NH₄)₃PO₄ is soluble in water.
Why This Matters:
This solubility information is important in contexts like precipitation reactions and determining whether a solution will contain free ions. When (NH₄)₃PO₄ dissolves in water, it dissociates completely into:
$$
3NH_4^+ + PO_4^{3-}
$$
These free ions in solution make the compound effective in various chemical reactions and industrial uses, such as fertilizers.
Conclusion:
Both ions in the compound are considered soluble, and the entire compound dissolves in water. Hence, the correct answer is:
✅ a. Both ions soluble