The basicity of ${\text{H}}_{\text{3}}{\text{PO}}_{\text{4}}$  in the reaction $\to$ ${\text{H}}_{\text{3}}{\text{PO}}_{\text{4}}+\text{NaOH}\stackrel{}{\to }{\text{NaH}}_{\text{2}}{\text{PO}}_{\text{4}}+{\text{H}}_{\text{2}}\text{O}$  is

Basicity : The number of replaceable hydrogen present in an acid when it reacts to form a salt. In NaH₂PO_4, still, 2 replaceable hydrogens are left

$\therefore$  Basicity of $H_{3}P{O}_4$  is 1.