Chem 1B
Dr. Abel
1 Chapter 15 – Applications of Aqueous Equilibria
15.1 The Common Ion Effect
A. Acid/Base solutions
1. What is the effect of adding a common ion (one already present in the
solution) to an acid or base solution?
2. Ex. A 0.050 M solution of benzoic acid (HC7H5O2; Ka = 6.5 x 10−6) has pH
= 2.74 and 3.6% dissociation. Calculate the new pH and percent
dissocation if 0.025 moles of sodium benzoate (NaC7H5O2) is added to 500.0
mL of 0.050 M benzoic acid. Assume no change in volume.
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Dr. Abel
2 15.2 Buffers
A. Buffered solutions
B. Calculations
1. Calculate the pH:
a. of a buffer solution consisting of 0.50 M HC2H3O2 and 0.50 M NaC2H3O2
b. After adding 0.020 mol of solid NaOH to 1.0 L of the buffer solution in part
a
c. After adding 0.020 mol of HCl to the 1.0 L of the buffer solution in part a.
Ka = 1.8 x 10-5 (assume no volume changes)
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Dr. Abel
3 C. How does a buffer work?
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Dr. Abel
4 D. Henderson-Hasselbalch equation
1. Derivation:
2. Ex. A buffer contains 0.50 M HNO2 and 0.25 M KNO2. What is the pH of
the solution? The Ka for nitrous acid is 4.4 x 10-4.
3. Ex. A buffer is created by mixing 25.0 mL of 0.50 M NH4Cl with 25.0 mL
of 0.50 M NH3 (Kb = 1.8 x 10-5). What is the pH of the buffer solution?
E. Calculating pH after adding acid/bases to buffers using H-H equation
1. Ex: A buffer is created by mixing 25.0 mL of 0.50 M HNO2 with 25.0 mL
of 0.50 M KNO2. What is the pH of the solution if 1.0 mL of 0.75 M HCl are
added to the buffer?
1) What is in the solution before any reaction, equilibrium or
otherwise, occurs?
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Dr. Abel
2) Are there any reactions other than equilibrium that can occur?
3) Solve equilibrium problem (use H-H equation!)
2. Ex: A buffer is created by mixing 25.0 mL of 0.50 M NH4Cl with 25.0
mL of 0.50 M NH3. What is the pH of the solution if 1.0 mL of 1.2 M NaOH
are added to the buffer?
5 Chem 1B
Dr. Abel
15.3 Buffering Capacity
A. Definition -
6 Chem 1B
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B. Preparing a Buffer
C. Buffer Prep Examples
Table 1: Selected Ka Values at 25°C
Acid
Ka
-3
H3PO4
7.6 x 10
-8
H2PO4
6.2 x 10
2-13
HPO4
2.1 x 10
-5
HC2H3O2
1.8 x 10
+
-10
NH4
5.8 x 10
1. What would be an appropriate buffer system to maintain a pH of 7.30?
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Dr. Abel
2. How would you prepare 0.500 L of an acetate buffer with a pH of 4.00
where the final concentration of the acid is 0.500 M using 1.0 M HC2H3O2
and solid sodium acetate?
3. How would you prepare 0.500 L of an acetate buffer with a pH of 4.00
where the final concentration of the acid is 0.500 M using 1.0 M HCl and
solid sodium acetate?
8 Chem 1B
Dr. Abel
9 4. How would you prepare 0.500 L of buffer with a pH of 4.00 where the final
acetic acid concentration is 0.500 M using 1.0 M HC2H3O2 and 1.0 M NaOH?
15.4 Titrations and pH Curves
A. Titrations –
B. pH Curves (titration curves) –
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Dr. Abel
C. Strong Acid-Strong Base Titrations
1. Example:
2. Net ionic equation:
3. What is present at the equivalence point?
4. What is the pH at the equivalence point?
5. Titration curve of a strong acid titrated with a strong base
pH Volume of Base Added (mL) 10 Chem 1B
Dr. Abel
6. Titration curve – Strong base titrated with a strong acid
pH Volume of Acid Added (mL) D. Weak Acid-Strong Base Titration
1. Example –
2. Net ionic –
3. What is present at the equivalence point?
4. What is the pH at the equivalence point?
5. Weak acid titrated with a strong base
pH Volume of Base Added (mL) 11 Chem 1B
Dr. Abel
6. Buffer region –
E. Diprotic Acid-Strong Base Titration
1. Example –
2. Overall Net ionic –
3. Protons Transferred in Two Steps:
12 Chem 1B
Dr. Abel
4. Diprotic acid titrated with a strong base
pH Volume of Base Added (mL) F. Weak Base-Strong Acid Titration
1. Example –
2. Net Ionic Equation –
3. What is present at the equivalence point?
4. What is the pH at the equivalence point?
5. Weak base titrated with a strong acid
pH Volume of Base Added (mL) 13 Chem 1B
Dr. Abel
F. Calculation of Ka
14 Chem 1B
Dr. Abel
15 15.5 Titrations –(Examples)
A. Examples –
1. 40.0 mL of HSCN 0.100 M is titrated with 0.100 M NaOH. The titration
curve is shown below. Use the curve to answer part c
a) Write the balanced equation:
b) Write the net ionic equation:
c) What is the Ka for the acid?
Titration of a HSCN with NaOH
14
12
10
0
8
pH
6
4
2
0
10
20
30
40
50
60
Volume of NaOH added (mL)
70
80
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Dr. Abel
16 2. Calculate the pH during the titration of 8.00 mL of 0.15 M NH3 (Kb = 1.8 x 10-5)
after adding the following volumes of 0.10 M HCl:
a) 0.00 mL
b) 3.00 mL
c) 6.00 mL (half-way to equivalence point)
d) 12.00 mL (equivalence point)
[you may need more than 1 page for these calculations!]
Chem 1B
Dr. Abel
17 3. Calculate the pH during the titration of 20.0 mL of 0.200 M HBrO (Ka=2.3x10-9)
with 0.100 M NaOH at the following points:
a. Before any base is added
b. Halfway to the equivalence point
c. At the equivalence point
d. When the moles of OH- are twice the moles of HBrO originally present.
[you may need more than 1 page for these calculations!]