Question

Give reasons for the following : Orthophosphoric acid, H₃PO₄, is tribasic, but phosphorous acid, H₃PO₃, is dibasic.

Answer 1

In H3PO3 two hydrogen atoms are attached through oxygen and one is directly attached to phosphorus

Only the hydrogens attached to oxygen can give H+

but not the hydrogen which is directly attached to the phosphorus

So it is dibasic

and in H3PO4 three hydrogens are attached to oxygen which give the H+ easily

so it is tribasic

Hope it helps you