Question

What is the basicity of orthophosphoric acid?
(A) One
(B) Two
(C) Three
(D) Four

Answer 1

As we know that, Lewis base is used to define the donation of lone pairs of electrons to any electron deficient compound. In this given compound, this term is only applicable for oxygen.

Complete answer:
Orthophosphorus acid is represented by the formula ${H_3}P{O_3}$ and represented by the structure as-

It is a Lewis acid because the phosphorus element in this compound is electron deficient due to $+ 3$ oxidation state and therefore, it accepts 2 lone pairs of electrons from 2$$$$OH groups which are directly attached to the phosphorus. Thus, the basicity of orthophosphoric acid is two because by donating electrons, oxygen carries positive charge so it attracts the electrons from hydrogen.
So now, we can define the term basicity as-
The basicity of the various acids, that is the number of replaceable ${\rm{H}}$ atoms, can be found by drawing the structure and counting the number of ${\rm{OH}}$ groups.

Hence, the correct option is option (B).

Additional information:
Both the above products are members of two series of oxoacids of phosphorus.
(a) The phosphoric series of acids, in which the oxidation state of ${\rm{P}}$ is $+ 5$, and in which the compounds have oxidizing properties.
(b) The phosphorous series of acids, in which the oxidation state of ${\rm{P}}$ is $+ 3$ , and in which the compounds have reducing properties.

Note: In all of the above acids, ${\rm{P}}$ is four coordinated and tetrahedrally surrounded wherever possible. ${\rm{p\pi - p\pi }}$ back bonding gives rise to ${\rm{P = O}}$ bonds. The hydrogen atoms in ${\rm{OH}}$ groups are ionisable and are acidic, but the ${\rm{P - H}}$ bonds found in phosphorous acids have reducing, not acidic.