CHEM1312H - pH of Salts that Yield Acidic Solutions and Solutions of Salts that Yield Amphoteric Species

salts that yield acidic solutions: weak bases and strong acids

-salts of weak bases and strong acids hydrolyze giving a solution with a pH less than 7

-the anion will become a spectator ion (conjugate base of a strong acid), while the cation from the weak base will donate a proton to the water forming a hydronium ion

salts that yield acidic solutions: hydrated cations

-hydrated cations of small, highly charged metal ions (with a 2+ charge or higher), such as Al³⁺, or Fe³⁺ yield acidic solutions

-small highly charged metal cations are lewis acids

-the ions of alkali and alkaline earth metals are large and not highly charged so they do not react with water and do not affect pH

salts from a weak base and a weak acid

-if the cation is either the conjugate acid of a weak base or a small, highly charged metal ion and the anion is the conjugate base of a weak acid

-to determine the overall pH of a solution containing one of these salts you will compare the value of K_a of the conjugate acid and K_b of the conjugate base

-if K_a > K_b the solution is acidic and if K_b > K_a, the solution will be basic, if K_a ~ K_b the solution will be neutral

salts that yield amphoteric species

-salts consisting of a cation from a strong base and an anion which is the conjugate base of a polyprotic acid 9H₃PO₄, H₂CO₃, H₂SO₃ etc) may yield amphoteric species (such as H₂PO₄^2-, HCO₃^-, and HSO₃^-)

-to determine the overall pH of this salt solution containing one of these salts you will compare the K_a of the acid and the K_b of the base

-if K_a > K_b the solution is acidic and if K_b > K_a the solution will be basic