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Saturday, November 3, 2018

Amphoteric salts hydrolysis:

Example of amphoteric salts  NaHS, NaHCO3, Na2HPO4, NaH2PO4

(A) HCO3- act as conjugate acid as well as conjugate base:

Both reaction will support each other extent of hydrolysis and extent of dissociation is same.

(B) Here H2PO4- and HPO4-2 are amphoteric anions. The pH of amphoteric salts anions is independent of concentration of salts.

Here HPO4-2 is conjugate base of H2PO4- and H3PO4 is conjugate acid of H2PO4-Similarly PO4-3 is conjugate base of HPO4-2 and H2PO4-1 is conjugate  acid of HPO4-2

When these salts are dissolved in water [H3O+] concentration can be determined as;

ILLUSTRATIVE EXAMPLE: Calculate pH of solution of

(1) 100 ml 0.1M H3PO4 + 100 ml 0.1M NaOH.

(2) 100 ml 0.1M H3PO4 + 200 ml 0.1M NaOH.

(3) 100 ml 0.1M H3PO4 + 300 ml 0.1M NaOH.

(4) 100 ml 0.1M H3PO4 + 400 ml 0.1M NaOH.



  1. Pls elaborate how NaHs is an amphoteric salt.

    1. NaHS is amphoteric salt it is ionised to give (NaHS--> Na+ + HS-) HS- ion which can donate H+ to form (S-2) sulphide ion as well as it can also accept H+ to form H2S hence NaHS is an amphoteric salt.