Explain why 'orthophosphoric acid, `H_3 PO_4`, is tribasic but phosphorus acid, `H_3 PO_3`, is dibasic'.

Explanation

`underset((H_(3)PO_(4)))(HO-overset(O)overset(||)underset(H)underset(|)(P)-OH)`
` underset((H_(3)PO_(3)))(HO-overset(P)overset(||)underset(H)underset(|)(P)-OH)`
Only those hydrogens are replaceable that are attached to oxygen an contribute to basicity. From the structures it is clear that orthophosphoric acid, `H_3 PO_4`, is tribasic but phosphorous acid, `H_3 PO_3`, is dibasic.