Unit 12
Weak Acids and Weak Bases
What makes a Weak Acid?
• Weak acids (covalently bonded H compounds)
are ionized by water less than 100%
• HBrO + H2O  H3O+ + BrO• According to Bronsted-Lowry, hypobromous
acid is the acid and the hypobromite ion is the
conjugate base.
What makes a Weak Base?
• Weak bases are the conjugate bases of the
weak acids (the anions) or they are the
organic bases—the amines
• The amines are derivatives of ammonia
• NH3 + H2O   NH4+ + OH• For Bronsted-Lowry, ammonia is the base
(proton acceptor) and the ammonium ion is
the conjugate acid (proton donor)
Weak: Don’t ionize 100%
• Bronsted-Lowry model for Acid-Base
Reactions
• HA + H2O D H3O+ + A• HA acid and A- conjugate base
• B + H2O D BH+ + OH• B base and BH+ conjugate acid
Conjugate Acid/Base
Conjugate Acid
Conjugate Base
HF
F-
HSO4-
SO42-
NH4+
NH3
Examples
• Label the acid, base, conjugate acid, and
conjugate base for #2 on page 374
• Determine it if substance would act as an
acid or base for #4 on page 374
• **remember acids lose an H+ and bases
gain an H+
Examples
• On page 375 let’s look at number 26
Homework 1
• Page 374 (1,3,5, 25)
Equilibrium Expression
• For HA + H2O  H3O+ + A• Ka = [H+] [A-] / [HA]
• For Base + H2O  BH+ + OH-
• Kb = [BH+ ] [OH-] / [Base]
EQUILIBRIUM
• Weak acids and bases set up an
equilibrium situation.
• EQUILIBRIUM: the rate of the forward rxn
= rate of the reverse rxn
• K is the equilibrium constant
• Ka = [H+]*[A-] / [HA]
• Kb = [BH+] * [OH-] / [B]
Weak Solutions
• If you put a weak acid in water, the water
pulls H+ from the acid because the
covalent bond is weaker in the acid.
• [H+] = [A-]
• If you put a weak base in water, the weak
base pulls an H+ from the water.
• [OH-] = [BH+]
Example: Weak Acid
• Calculate the pH of a 2M HBrO solution.
What is the % ionization?
Example: Weak Base
• What is the pH of a 3M solution of
ammonia, a weak base?
Example: Finding Ka
• Aspirin is a weak acid. A 0.1 M solution
has a pH = 2.24. What is the Ka?
Example: Finding Ka
• Saccharin is a weak acid. A 0.1 M
solution ionizes 22%. Calculate the
concentration of all species, the pH, and
Ka.
Example: Finding Kb
• Cocaine is a weak base. A 0.0010 M
solution has a pH of 9.7, what is the Kb?
Homework 2 & 3
• Page 375 33-41 odd
• Page 376 49, 53-58
Polyprotic Acids
• Water pulls off 1 H+ at a time.
• Ionization occurs in stages
• Water will be less effective at pulling off an
H+ from the negative anion at each step.
• The #H+ = # ionizations
Multiprotic Acids
• H3A  H+ + H2A• H2A-  H+ + HA2• HA2-  H+ + A3-
Ka1
Ka2
Ka3
Multiprotic Acid Example
• What is the pH and concentration of all
species present in a 1.5M solution of
phosphoric acid (in soft drinks)?
Homework 4
• Page 375 (45-48)
How to determine if the salts are
acidic, basic or neutral in water
solutions?
•
•
•
•
•
NaNO2
NaF
HCl
NaCN
HCO3-
Homework 5
• Page 376 (59, 60)