Question

The form of phosphorus in rock phosphate

• A. $\text{Ca}(\text{H}_2\text{PO}_4)_2$
• B. $\text{CaHPO}_4$
• C. $\text{Ca}_3(\text{PO}_4)_2$
• D. $3(\text{Ca}_3(\text{PO}_4)_2) \cdot \text{Ca}(\text{OH})_2$

Hint:

Because of its apatite structure, rock phosphate is highly insoluble in water. It is only effective as a direct fertilizer in highly acidic soils ($\text{pH} < 5.5$) that help dissolve the mineral structure.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Rock phosphate is a naturally occurring, insoluble mineral source used to manufacture soluble phosphate fertilizers or applied directly to strongly acidic soils.

Step 2: Detailed Explanation:
- The primary phosphorus-bearing minerals in rock phosphate belong to the apatite group.
- These minerals are highly insoluble, crystalline calcium phosphates with the general formula $\text{Ca}_{10}(\text{PO}_4)_6\text{X}_2$ (where $\text{X}$ can be $\text{F}^-$, $\text{OH}^-$, or $\text{Cl}^-$).
- Hydroxyapatite is represented chemically as $\text{Ca}_{10}(\text{PO}_4)_6(\text{OH})_2$.
- Expressed in oxides or grouped salts, this is written as $3(\text{Ca}_3(\text{PO}_4)_2) \cdot \text{Ca(\text{OH})_2$}.
- This complex, highly stable structure explains why rock phosphate has very low water solubility and requires acidic soil conditions to release orthophosphate ions.

Step 3: Final Answer:
The form of phosphorus in rock phosphate is $3(\text{Ca}_3(\text{PO}_4)_2) \cdot \text{Ca}(\text{OH})_2$.