28 Jan 2019
School
Department
Course
CHMA11 Lecture 10: Characterizing Acids and Bases
- Polyprotic Acids: Acids that can produce more than one molecule of hydronium when
they dissociate
o These acids are able to ionize in multiple steps
▪ Gives them multiple Acid Ionization constant (Ka) values
o Finding the pH of polyprotic acid solutions is thus slightly harder than in normal
acids
▪ Key fact is that the first Ka value will always be much larger than the
second
• Therefore we are able to use the first Ka value to get an initial pH
value
• We can then use the second Ka value to solve for the pH as if we
were in a weak acid equilibrium using ICE tables
▪ For strong polyprotic acids, the concentration of the acid initially will
equal the concentration of the acids once it undergoes its first
dissociation
• Therefore for these strong acids if we know the initial
concentration, we will know the concentration of it after it
undergoes its first disassociation
• We can then insert this concentration into the calculations for the
second ionization and solve for the hydronium concentration
using the second Ka value
o Will thus allow us to determine pH
- Classifying salt solutions as acidic, basic or neutral
o Since most salts will contain both a cation and anion, they are able to form
acidic, basic or neutral solutions when dissolved in water
o The pH of the solution depends on the specific cation and anion involved
o 4 potential possibilities
▪ 1: Salts in which neither the cation nor anion act as an acid or base will
form pH neutral solutions
• This involves salts in which the cation is the counter ion of a
strong base
• Also involves salts in which the anion is the conjugate base of a
strong acid
• These salts form neutral solutions
• Eg: NaCl, KBr
▪ 2: Salts in which the cation does not act as an acid and the anion acts as a
base will form basic solutions
• Involves salts in which the cation is an alkali metal