Question

What is the basicity of \[{{H}_{3}}P{{O}_{3}}\]?
\[\]

Answer 1

- Hint: tart by considering the basic definition of basicity and check the number of replaceable hydrogen atoms in Phosphorous acid, i.e. \[{{H}_{3}}P{{O}_{3}}\].

Complete step by step answer:
Basicity is defined as the number of electron pairs it can easily donate. Basicity depends on the following factors –
Electronegativity
Size
Number of Resonance structures
The structure of \[{{H}_{3}}P{{O}_{3}}\] is as given below-

As we can see, \[{{H}_{3}}P{{O}_{3}}\] has two Hydrogens attached to Oxygen.
Oxygen has a high electronegativity (= 3.44) and therefore tends to pull the electron pair (electrons shared between Oxygen and Hydrogen) towards itself, making the Hydrogen attached to it highly positive. So, when \[{{H}^{+}}\]ion is removed from \[{{H}_{3}}P{{O}_{3}}\], negative charge on oxygen is stabilized (firstly, because of its higher electronegativity, secondly, due to its resonating structures). There are 4 resonating structures of phosphite ion \[HPO_{3}^{2-}\]. Therefore, it is a stable compound.

From the structure of \[HPO_{3}^{2-}\], we can see that it has 2 free electron pairs to donate.
Therefore, the basicity of \[{{H}_{3}}P{{O}_{3}}\] is 2.

Additional Information:
A compound basicity equal to 2 is also known as dibasic or diprotic. Similarly, basicity equal to 1 or 3 is known as mono-basic/protic and tri-basic/protic respectively.

Note: Basicity has many definitions.
Lewis Base Theory – “A Lewis base is any substance, such as the OH- ion, that can donate a pair of nonbonding electrons. A Lewis base is therefore an electron-pair donor”.
Bronsted-Lowry Theory – “A bases are defined as substances that dissociate in aqueous solution to give hydroxide ions”.
Arrhenius Theory – “A base is a compound that can dissociate in water to yield hydroxide ions”.