What is the basicity of `H_(3)PO_(4)` ?

Explanation

The structure of `H_(3)PO_(4)` is tetrahedral. It has three P-OH and one P = O bond as shown below : Since it has three ionizable P-OH bonds, therefore, `H_(3)PO_(4)` is tribasic as shown below :
`{:(H_(3)PO_(4) + H_(2)O,hArr,H_(3)O^(+) + H_(2) PO_(4)^(-)",",pK_(a_(1)) = "2.12 at 298 K"),(H_(2)PO_(4)^(-) + H_(2)O,hArr,H_(3)O^(+) + HPO_(4)^(2-),,pK_(a_(2))="7.2 at 398 K"),(HPO_(4)^(2-) + H_(2)O,hArr,H_(3)O^(+) + PO_(4)^(3-),,pK_(a_(3))="12.4 at 398 K"):}`
Since it is tribasic, it forms three series of salts with NaOH, viz., `NaH_(2)PO_(4), Na_(2)HPO_(4) and Na_(3)PO_(4)`.