Question

A waxy white solid is burnt in air (with an excess supply of air) in an extraordinarily exothermic reaction. The product of this reaction rapidly undergoes complete hydrolysis to yield an acid X. The acid X is:

• A. Hypophosphorus acid
• B. Phosphorous acid
• C. Fluorapatite or Phosphorite
• D. Orthophosphoric acid

Answer 1

The correct option is D Orthophosphoric acid

When Phosphorus is burnt in excess air (or oxygen), it gives a white solid:
$P_4\left(s\right)+5O_2\left(g\right)\to P_4{O}_{10}\Delta H=-2980kJ.mo{l}^{-1}$

On complete (and violent) hydrolysis, we get orthophosphoric acid:
$P_4{O}_{10}+6H_{2}O\to 4H_{3}P{O}_4$