1. Arrhenius: A base furnishes a hydroxide ion (OH-) in solution.
Weak Bases in Water:
Arrhenius: A base furnishes a hydroxide ion (OH-) in solution.
Bronsted-Lowry: A base is a proton acceptor
NH3
C6H5NH2
(CH3)2NH
+ H2O  NH OH-
base acid conj acid conj. base
+ H2O  C6H5NH OH-
base acid conj acid conj. base
+ H2O  (CH3)2NH OH-
base acid conj acid conj. base
The OH- ion tells us that these are Kb problems.
Also: KaKb=Kw and 25oC = 1 x10-14

2. Given: Kb for trimethylamine (CH3)3N is 6.4 x 10-5
Problem: Calculate the pH of a 0.05 M (CH3)3N solution.
Hydrolysis Eq. Expression:
(CH3)3N + H2O  (CH3)3NH OH- I. C. E.
-x x x
x x x
try dropping to simplify the math
What does x represent?
[OH-]
x = 1.79 x 10-3
pOH = -log[1.79 x 10-3] = 2.75
pH = = 11.25

3. Given: Kb for trimethylamine (CH3)3N is 6.4 x 10-5
Problem: Calculate the pH of a 0.05 M (CH3)3N solution.
Hydrolysis Eq. Expression:
(CH3)3N + H2O  (CH3)3NH OH- I. C. E.
-x x x
x x x
try dropping to simplify the math
What does x represent?
[OH-]
x = 1.79 x 10-3