why some texts calculate the pH of KHA considering HA- only like a "weak monoprotic acid solutiom HA " and not an ampholyte ??
Because some people have no idea what they are doing. The answer given is wrong, and it is not that difficult to prove.
He calculated:
[H
+] = 0.000265 M
so (from the dissociation stoichiometry):
[P
2-] = 0.000265 M
[HP
-] = 0.01763 - 0.000265 M
We have
[tex]K_{a1} = \frac {[H^+][HP^-]}{[H_2P]}[/tex]
so
[tex][H_2P] = \frac {[H^+][HP^-]}{K_{a1}}[/tex]
or
[tex][H_2P] = \frac {0.000265 \times (0.01763 - 0.000265)}{10^{-2.89}} = 0.00357[/tex]
Now, total (analytical) concentration of phthalate is
[tex]C_A = [H_2P]+[HP^-]+[P^{2-}][/tex]
or (using the numbers calculated)
[tex]C_A = 0.00357+(0.01763 - 0.000265)+0.000265 = 0.0212[/tex]
So the phthalate concentration calculated from the equilibrium concentrations is 0.0212 M when we know it should be 0.01763 M. Apparently something is wrong.
Try to calculate pH from the formula for ampholyte and apply the same reasoning to see if the answer leads to contradiction (or not).