Question

What is the acidic strength order of $H_{3}P{O}_2$, $H_{3}P{O}_3$, $H_{3}P{O}_4$ ? Why ?

Answer 1

In oxoacids, phosphorus is tetrahedrally surrounded by other atoms. All these acids contain at least one P=O bond and one P–OH bond.

• Generally, the acidic strength of oxoacids depends on the oxidation number of the central atom. Higher the oxidation state, the acidic character will be high.​So, the expected order is $H_{3}P{O}_4$ > $H_{3}P{O}_3$>$H_{3}P{O}_2$
  But the actual order is : $H_{3}P{O}_2$>$H_{3}P{O}_3$>$H_{3}P{O}_4$
  

• This can be explained based on the number of $O{H}^{-}$, groups attached to the central P-atom. With an increasing number of $O{H}^{-}$ groups on the central P-atom, the acidic strength decreases. As the number of $O{H}^{-}$ groups increases, the strength of the conjugate base formed increases due to a greater proton accepting tendency. Therefore, the strengths of corresponding acids decrease accordingly.
• Acidic strength order : $H_{3}P{O}_2>H_{3}P{O}_3>H_{3}P{O}_4$