Question

Why $H_{3}P{O}_4$ is a tribasic and $H_{3}P{O}_3$ is a dibasic acid?

Answer 1

$H_{3}P{O}_4$, phosphoric acid, can be written $O=P(OH{)}_3$. It has three acidic protons (attached to oxygen) and is therefore tribasic. Its salts are called phosphates.

But $H_{3}P{O}_3$, phosphoric acid, has one of the hydrogens directly bound to phosphorus as $O=P-H(OH{)}_2$. This means that only two hydrogens are available to act as acids. The salts of phosphoric acid are called phosphonates.

There is also an $H_{3}P{O}_3$, phosphorous acid $P(OH{)}_3$, that is tribasic. The salts of phosphorous acid are called phosphites.

$H_{3}P{O}_2$ is monobasic acid, $H_{3}P{O}_3$ is dibasic acid and $H_{3}P{O}_4$ is tribasic acid. In the case of acid of phosphorus acidic nature depend on how much OH is present in structure because it shows acidic nature due to only these H. The H which is directly attached to phosphorus shows a reducing nature of acid only.