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Weak Bases in Water:
Arrhenius: A base furnishes a hydroxide ion (OH-) in solution.
Bronsted-Lowry: A base is a proton acceptor
NH3
base
+ H2O  NH4+ + OHacid
C6H5NH2
base
base
conj. base
+ H2O  C6H5NH3+ + OHacid
(CH3)2NH
conj acid
conj acid
conj. base
+ H2O  (CH3)2NH2+ + OHacid
conj acid
conj. base
The OH- ion tells us that these are Kb problems.
[base H ][O H ]
Kb 
[base]
Also: KaKb=Kw and Kw @ 25oC = 1 x10-14
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:


[base H ][O H ]
Kb 
I.
[base]
C.
E.
x2
Kb 
0.05  x 2
x
-5
6.4 x 10 
0.05  x
x = 1.79 x
10-3
(CH3)3N + H2O  (CH3)3NH+ + OH0.05
-x
0.05 - x
0
+x
x
try dropping to simplify the math
What does x represent?
pOH = -log[1.79 x 10-3] = 2.75
[OH-]
pH = 14 -2.75 = 11.25
0
+x
x
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:


[base H ][O H ]
Kb 
I.
[base]
C.
E.
(CH3)3N + H2O  (CH3)3NH+ + OH0.05
-x
0.05 - x
0
+x
x
x2
Kb 
0.05  x
2
x
6.4 x 10-5 
0.05  x
x = 1.79 x
10-3
try dropping to simplify the math
What does x represent?
[OH-]
0
+x
x