Chapter 17:
Additional Aspects of Acid-Base Equilibria
or
Double Awesome!!
Titrations and Buffers and
Equilibria!!!!
I. Buffer Solutions
Titrations kick
Acid!!!
A. Common Ion Effect in Acid and
Base Equilibria
B. Common-Ion Effect and Buffered
Solutions
II. Titrations in Detail
A. Strong Acid with a Strong Base Titrations
B. Acid and Base Indicators
C. Weak Acid with a Strong Base and
Weak Base with a Strong Acid Titrations
D. Polyprotic Acid with a Strong
Base Titrations
1
I. Buffer Solutions
A. Common Ion Effect in Acid and Base Equilibria
HC2H3O2(aq) + H2O(l) ⇌ C2H3O2-(aq) + H3O+(aq)
Add HCl(aq) -
Add NaC2H3O2(aq)-
2
B. Common-Ion Effect and Buffered Solutions
1.000 L of H2O
Principal Species –
1. Add 1.0x10-3 moles of NaOH -
2. Add 1.0x10-3 moles of HCl -
1.000 L of 0.100 M HC2H3O2 and 0.100 M NaC2H3O2(aq)
Principal Species –
1. Add 1.0x10-3 moles of NaOH –
Stoichiometric Calculation (use mmol)
3
Equilibrium Calculation (use concentrations)
4
2. Add 1.0x10-3 moles of HCl
Stoichiometric Calculation (use mmol)
H3O+(aq) + C2H3O2-(aq)  HC2H3O2(aq) + H2O(l)
Initial 1.0 mmol
100. mmol
100. mmol
Rxn -1.0 mmol
-1.0 mmol
+1.0 mmol
After
0 mmol
99.|0 mmol
101.|0 mmol
Rxn
Equilibrium Calculation (use concentrations)
⇌ H3O+(aq) + C2H3O2-(aq)
0.099|0 M
+x
+x
x
0.099|0 M + x
HC2H3O2(aq) + H2O(l)
I.
0.101|0 M
C.
-x
E.
0.101|0 M - x
Ka =
(𝑥 )(0.099 + x)
( 0.101 − x)
=
(𝑥 )(0.099)
( 0.101)
= 1.8x10-5
x = [H3O+] = 1.8|36x10-5 M
Check Assumption
1.8|36x10−5 M
0.099 M
𝑥100% =1.8|54x10-2 %
pH = 4.73|61 = 4.74
5
Comparison of Water and HC2H3O2/C2H3O2- Solution
Water
- added 1.0 x10-3 moles OH-, pH increased 4 pH units
- added 1.0 x10-3 moles H3O+, pH decreased 4 pH units
HC2H3O2/C2H3O2- Solution
- added 1.0 x10-3 moles OH-, pH increased 0.01|35 pH units
- added 1.0 x10-3 moles H3O+, pH decreased 0.00|39 pH units
Solutions with relatively equal concentrations of a weak
acid and its conjugate base (or vice versa) are called
BUFFERS
BUFFERS -
resist large changes in pH on addition of
strong acids or bases
6
Calculating pH of Buffer Solutions
HC2H3O2(aq) + H2O(l)
I.
0.100 M
C.
-x
E.
0.100 M - x
⇌ H3O+(aq) + C2H3O2-(aq)
0.100 M
+x
+x
x
0.100 M + x
Calculating pH of a buffer once an acid or base has been added?
Henderson-Hasselbalch Equation
Added acid or base will slightly change concentrations of buffer
components, but shift towards new equilibrium will still be small
relative to the concentrations of the buffer components
7
Henderson-Hasselbalch Equation Cont’d
Generic Form -
pH =pKa +log (
[𝐶𝑜𝑛𝑗𝑢𝑔𝑎𝑡𝑒 𝐵𝑎𝑠𝑒 ]𝑖𝑛𝑖𝑡𝑖𝑎𝑙
[𝐴𝑐𝑖𝑑 ]𝑖𝑛𝑖𝑡𝑖𝑎𝑙
Is the H-H Equation Always Valid? -
When can we use the H-H Equation?
8
)
Example Problem
The H2PO4- - HPO42- buffer combination plays a role in
maintaining the pH of blood.
a.) Write equations to show how a solution containing
these ions can function as a buffer.
b.) Verify that this buffer is most effective at pH of 7.2.
c.) Calculate the pH of a buffer made by adding 0.100
moles of HCl to 1.00 L of a 0.150 M Na2HPO4
solution.
d.) What is the buffer capacity of this buffer?
9
Example Problem Cont’d
10
II. Titrations in Detail
Why Titrate -
Powerful analytical tool used to determine
concentration of unknown acid or base
Types of Titrations
A. Strong Acid with a Strong Base (or vice versa)
C. Weak Acid with a Strong Base
Weak Base with a Strong Acid
D. Polyprotic Acid with a Strong Base
Equivalence Point – Point at which just enough base
has been added to react with all
of the acid present
How do we know we have reached the equivalence point?
1. Use an acid-base indicator
11
2. Use a pH meter and plot pH as a function of added acid
or base (Titration Curve)
pH meter – measures activity of H3O+, provides a “better”
estimation of the equivalence point
Four Regions of the Titration Curve
1. Before any base is added
2. After some base is added but not enough to reach the
equivalence point
3. When just enough base has been added to react with all
of the acid
4. When excess base has been added
12
A. Strong Acid with a Strong Base Titration
A 50.0 mL sample of 0.200 M HNO3 is titrated with
0.100 M NaOH. What is the pH of the solution after 0, 20,
100, and 150 mL of NaOH have been added?
13
Strong Acid with a Strong Base Titration Cont’d
14