Question

The form of phosphorus in rock phosphate is:

• A. Ca(H$_2$PO$_4$)$_2$
• B. CaHPO$_4$
• C. Ca$_3$(PO$_4$)$_2$
• D. 3(Ca$_3$(PO$_4$)$_2$)·Ca(OH)$_2$

Hint:

Rock phosphate primarily contains insoluble tricalcium phosphate, which is processed into soluble forms for agricultural use.

Answer 1

The Correct Option is D

The major form of phosphorus in rock phosphate is tricalcium phosphate combined with calcium hydroxide, represented as 3(Ca$_3$(PO$_4$)$_2$)·Ca(OH)$_2$. This form is generally insoluble in water but becomes available to plants in acidic soils or through treatment with acids to make phosphate fertilizers.

Answer 2

Elimination approach:
\( \text{Ca(H}_2\text{PO}_4\text{)}_2 \) (monocalcium phosphate) and \( \text{CaHPO}_4 \) (dicalcium phosphate) are water-soluble or partly soluble products made by acid-treating rock phosphate, not the natural mineral itself. \( \text{Ca}_3(\text{PO}_4)_2 \) (tricalcium phosphate) is a simplified formula that does not reflect the true apatite structure found in nature. Rock phosphate actually occurs as apatite, whose composition is written as \( 3(\text{Ca}_3(\text{PO}_4)_2)\cdot\text{Ca(OH)}_2 \), calcium phosphate associated with calcium hydroxide, which is sparingly soluble and needs acidulation before plants can use it.