When a small amount of HCl or NaOH is added to a solution of Na2HPO4, the pH of the solution barely changes. Based on these observations, all of the following are true concerning the compound Na2HPO4, except
A) Na2HPO4 is able to accept extra hydrogen ions from the HCl.
B) Na2HPO4 is able to donate hydrogen ions to the OH- from NaOH.
C) Na2HPO4 adsorbs excess H+ and OH- directly onto the surface of its crystalline structure.
D) Na2HPO4 is a salt formed from reacting a strong base with a weak acid.
E) Na2HPO4 acts as a buffer.

Question

Quizplus · Accepted Answer

Na2HPO4 is a salt of a weak acid (H2PO4-) and a strong base (NaOH), therefore it is not able to donate hydrogen ions to the OH- from NaOH (choice B) and it is not a buffer (choice E). The fact that a small amount of HCl or NaOH barely changes the pH indicates that Na2HPO4 has the ability to adsorb excess H+ and OH- directly onto the surface of its crystalline structure (choice C). However, Na2HPO4 cannot accept extra hydrogen ions from the HCl (choice A) because HCl is a strong acid that almost completely dissociates in water, and Na2HPO4 is not capable of buffering against the strong acid.

When a Small Amount of HCl or NaOH Is Added