The number of non-ionisable protons present in the product $\mathrm{B}$ obtained from the following reaction is___${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}+{\mathrm{PCl}}_3\to {\mathrm{C}}_2{\mathrm{H}}_5\mathrm{Cl}+\mathrm{A}$

$\mathrm{A}+{\mathrm{PCl}}_3\to \mathrm{B}$

${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}+{\mathrm{PCl}}_3⟶{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{Cl}+{\mathrm{H}}_3{\mathrm{PO}}_3$

${\mathrm{H}}_3{\mathrm{PO}}_3+{\mathrm{PCl}}_3⟶{\mathrm{H}}_4{\mathrm{P}}_2{\mathrm{O}}_5+\mathrm{HCl}$

A is ${\mathrm{H}}_3{\mathrm{PO}}_3$(Phosphorous acid)

The basicity of A is two

B is ${\mathrm{H}}_4{\mathrm{P}}_2{\mathrm{O}}_5$(Pyrophosphorous acid)

Structure of ${\mathrm{H}}_4{\mathrm{P}}_2{\mathrm{O}}_5$ is as given below

Hydrogen which are directly attached to the phosphorous in the above structure are non-ionisable.

The Number of non-ionisable hydrogen atoms are two in the above structure.