Acidic and Basic Salts A salt is an ionic compound that contains a cation other than H+ and an anion other than OH-. CaSO4, BaI2 When a salt undergoes hydrolysis, the resulting aqueous solution may be acidic, basic, or remain neutral. To determine the acidity of the resulting solution, you must look at the resulting cations and anions.

Acidic and Neutral Cations Cations can be either acidic or neutral but not basic. Cations producing an acidic solution are NH4+, and the transition metal ions which can form complex ions (Zn, Cu, Al, Ni). NH4+(aq) H+(aq) + NH3(aq) Al3+(aq) + 6H2O(l) Al(H2O)63+(aq) Al(H2O)63+(aq) H+(aq) + Al(H2O)5(OH)2+(aq)

Note the following cations which are spectators. Li+, Na+, K+, Ca2+, Ba2+, Sr2+ These cations are found in strong bases. If a strong base wants to completely dissociate, why would its cation want to undergo hydrolysis to reform the base? These cations will not undergo hydrolysis. There are no basic cations.

Basic and Neutral Anions Anions can be either basic or neutral but not acidic. Anions producing a basic solution are C2H3O2-, F-, CO32-, PO43-, CN-, NO2-, and S2-. C2H3O2- (aq) + H2O(l) HC2H3O2 (aq) + OH-(aq)

Note the following anions which are spectators. Cl-, Br-, I-, NO3-, ClO4-, SO42- These species are conjugate bases of strong acids. Remember, the stronger the acid, the weaker its conjugate base. If a molecule wants to completely ionize, why would its anion want to undergo hydrolysis?

Acidic, Basic, or Neutral Salt? LiClO4 - acidic, basic, or neutral? Is the cation, Li+, a metallic ion of Group I or Group II? Yes, as it is found in a strong base (LiOH), and will be a spectator.

Is the anion, ClO4-, the conjugate base of a strong acid, HClO4? Yes, therefore it will be a spectator. Both cation and anion are spectators, therefore LiClO4 is a neutral salt.

Cu(NO3)2 - acidic, basic, or neutral? Is the cation, Cu2+, a metallic ion of Group I or Group II? No but Cu2+ being a transition metal will hydrolyze producing an acidic solution. Cu2+ with vacant orbitals will act as a Lewis acid and will be an electron acceptor.

Cu2+(aq) + 4H2O(l) Cu(H2O)42+(aq) Cu(H2O)42+(aq) will in turn act as an Arrhenius or Bronsted-Lowry acid. Cu(H2O)42+(aq) H+(aq) + Cu(H2O)3(OH)+(aq) Is the anion, NO3-, the conjugate base of a strong acid, HNO3? Yes, therefore it will be a spectator.

The salt will be acidic due to the Cu2+.

NaH2PO4 - acidic, basic, or neutral? Is the cation, Na+, a metallic ion of Group I or Group II? Yes, as it is found in a strong base (NaOH), and will be a spectator.

Is the anion, H2PO4-, the conjugate base of a strong acid, H3PO4? No because H3PO4 is a weak acid. Before saying that NaH2PO4 is a neutral salt, you must ask is H2PO4- amphiprotic? If the anion, H2PO4-, is not amphiprotic, then the solution will be basic!

If the anion, H2PO4-, is amphiprotic then you must determine if the salt is acidic or basic! H2PO4- is amphiprotic, therefore you must compare the Ka of H2PO4- to the Kb of H2PO4-. If H2PO4- (dihydrogen phosphate) acts as an acid the following is true. H2PO4-(aq) H+(aq) + HPO42-(aq) Ka(H2PO4-) = 6.2 × 10-8

If H2PO4- acts as a base the following is true. H2PO4-(aq) + H2O(l) H3PO4(aq) + OH-(aq) The Kb for H2PO4- is not given in a table of Dissociation Constants for Acids and Bases at 25°C.

However, it can be calculated from the relationship between an acid and its conjugate base given by, Ka × Kb = Kw and its hydrolysis equation. H2PO4-(aq) + H2O(l) H3PO4(aq) + OH-(aq) Ka(H3PO4) = 7.5 × 10-3, therefore

Kw Ka 10-14 Kb = = = 1.3 × 10-12 7.5 × 10-3 Ka > Kb, therefore NaH2PO4 is an acidic salt.

Acidic, Basic, Neutral Salt WrapUp Is the cation a metallic ion from Group I or II? If yes, the cation will be a spectator ion. If no, it will most likely be acidic because the cation will be a transition metal. Is the anion the conjugate base of a strong acid? If yes, the anion will be a spectator ion.

If the anion is not a spectator ion, is it amphiprotic? If yes, then it can be either acidic or basic. If Ka > Kb, then the species is acidic. If Kb > Ka, then the species is basic. If no, then the salt will be basic.