Question

What is the basicity of ${{H}_{3}}P{{O}_{3}}$?

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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”.