1. Buffers and Acid/Base Titration

2. Reaction of Weak Bases with Water
The base reacts with water, producing its conjugate acid and hydroxide ion:
CH3NH2 + H2O  CH3NH3+ + OH- Kb = 4.38 x 10-4

3. Kb for Some Common Weak Bases
Many students struggle with identifying weak bases and their conjugate acids.What patterns do you see that may help you?
Base
Formula
Conjugate Acid Kb
Ammonia 
 NH3
 NH4+
 1.8 x 10-5 
 Methylamine
 CH3NH2
 CH3NH3+
 4.38 x 10-4 
 Ethylamine
 C2H5NH2
 C2H5NH3+
 5.6 x 10-4 
 Diethylamine
 (C2H5)2NH
 (C2H5)2NH2+
 1.3 x 10-3 
 Triethylamine
  (C2H5)3N
  (C2H5)3NH+
 4.0 x 10-4 
 Hydroxylamine
 HONH2 
 HONH3+  
 1.1 x 10-8 
 Hydrazine
H2NNH2 
H2NNH3+  
 3.0 x 10-6 
 Aniline
 C6H5NH2 
 C6H5NH3+  
 3.8 x 10-10 
 Pyridine
 C5H5N 
 C5H5NH+  
 1.7 x 10-9 

4. Reaction of Weak Bases with Water
The generic reaction for a base reacting with water, producing its conjugate acid and hydroxide ion:
B + H2O  BH+ + OH-
(Yes, all weak bases do this – DO NOT
endeavor to make this complicated!)

5. Buffered Solutions
A solution that resists a change in pH when either hydroxide ions or protons are added.
Buffered solutions contain either:
A weak acid and its salt
A weak base and its salt

6. Acid/Salt Buffering Pairs
The salt will contain the anion of the acid, and the cation of a strong base (NaOH, KOH)
Weak Acid
Formula
of the acid
Example of a salt of the weak acid
 Hydrofluoric 
HF 
 KF – Potassium fluoride 
 Formic 
 HCOOH 
 KHCOO – Potassium formate 
 Benzoic 
 C6H5COOH 
 NaC6H5COO – Sodium benzoate
 Acetic 
 CH3COOH 
 NaH3COO – Sodium acetate 
 Carbonic 
 H2CO3 
 NaHCO3 - Sodium bicarbonate
 Propanoic 
 HC3H5O2 
  NaC3H5O2  - Sodium propanoate
 Hydrocyanic 
 HCN 
 KCN - potassium cyanide 

7. Base/Salt Buffering Pairs
The salt will contain the cation of the base, and the anion of a strong acid (HCl, HNO3)
Base
Formula of the base
Example of a salt of the weak acid
Ammonia 
 NH3
 NH4Cl - ammonium chloride
 Methylamine
 CH3NH2
 CH3NH2Cl – methylammonium chloride
 Ethylamine
 C2H5NH2
 C2H5NH3NO3 -  ethylammonium nitrate
 Aniline
 C6H5NH2 
C6H5NH3Cl – aniline hydrochloride
 Pyridine
 C5H5N 
  C5H5NHCl – pyridine hydrochloride

8. Titration of an Unbuffered Solution
A solution that is
0.10 M CH3COOH
is titrated with
0.10 M NaOH

9. Titration of a Buffered Solution
A solution that is
0.10 M CH3COOH and
0.10 M NaCH3COO is titrated with
0.10 M NaOH

10. Comparing Results
Buffered
Unbuffered

11. Comparing Results Unbuffered Buffered
In what ways are the graphs different?
In what ways are the graphs similar?

12. Weak Acid/Strong Base Titration
Endpoint is above pH 7
A solution that is
0.10 M CH3COOH
is titrated with
0.10 M NaOH

13. Strong Acid/Strong Base Titration
Endpoint is at
pH 7
A solution that is
0.10 M HCl is titrated with
0.10 M NaOH

14. Strong Acid/Strong Base Titration
A solution that is
0.10 M NaOH is titrated with
0.10 M HCl
Endpoint is at
pH 7
It is important to recognize that titration curves are not always increasing from left to right.

15. Strong Acid/Weak Base Titration
A solution that is
0.10 M HCl is titrated with
0.10 M NH3
Endpoint is below
pH 7

16. Selection of Indicators