Question

Why does phosphorous acid form only two series of salts, even though the molecule contains three hydrogen atoms?

Short answer

Because \({{\rm{H}}_3}{\rm{P}}{{\rm{O}}_3}\)is a diprotic acid with two ionisable protons, it only forms two salt series.

Step-by-step solution

Determine of phosphorous acid

• An acid is defined as a substance that contributes or removes a proton\(({{\rm{H}}^ + })\)ion in solution, according to the Bronsted-Lowry idea.
• When weak acids are ionised, they lose a proton, which is represented by an equilibrium between the protonated and deprotonated forms of the acid.

Formation of phosphorous acid

\({{\rm{H}}_3}{\rm{P}}{{\rm{O}}_3}\)stands for phosphorus acid, which is a weak acid. The Lewis structure of phosphoric acid is as follows:

O is more electronegative than P in this situation. As a result, only the H atoms connected to the O atoms can be easily removed as protons. As a result,\({{\rm{H}}_3}{\rm{P}}{{\rm{O}}_3}\)is classified as a diprotic acid. The ionisation reactions that result from the loss of the two protons occur in a stepwise way, as shown below:

\(\begin{array}{l}{{\rm{H}}_3}{\rm{P}}{{\rm{O}}_3} \leftrightarrow {{\rm{H}}^ + } + {{\rm{H}}_2}{\rm{PO}}_3^ - \;\;\;\;\;\;\; - - - - - (1)\\{{\rm{H}}_2}{\rm{PO}}_3^ - \leftrightarrow {{\rm{H}}^ + } + {\rm{HPO}}_3^{2 - }\;\;\;\;\;\; - - - - - (2)\end{array}\)