Buffer Solutions Overview: Definitions, Examples, and Math
Overview/Topics
1.
Definition of Buffers
2.
Buffer Strengths
3.
Making Buffer Solutions
4.
Examples of Buffer Systems
5.
Buffer Math
6.
Henderson-Hasselbalch Equation (HHE)
Skills to Master
1.
HW 14 d
Read
OER 14.5-14.6
CHE 112 Spring 2021
Lecture 14 d – Buffer Solutions
Buffers
o
Solutions that are resistant to changes in pH
o
wa + salt of conjugate base
o
wb + salt of conjugate acid
Weak Acid
Salt of Weak Acid
Replace H with Metal Ion
Salt = anion of SA
cation of SB
How Buffers Work
HC
2
H
3
O
2
+ NaOH → NaC
2
H
3
O
2
+ H
2
O
Acid + Base = Salt + Water
NaC
2
H
3
O
2
+ HCl → HC
2
H
3
O
2
+ NaCl
Salt CB + Acid = Weak Acid + Salt
Equilibrium
HC
2
H
3
O
2
(aq) + H
2
O (l) ↔ H
3
O
+
(aq) + C
2
H
3
O
2
-
(aq)
You Try It:
Which of the following is a Buffer Systems?
(a)
H
2
SO
4
/ Na
2
SO
4
(b)
HF / KF
(c)
NH
3
/ NH
4
Cl
(d)
NaOH / KOH
(e)
HC
2
H
2
O
2
/ HF
To make a buffer system you could add which of the following to HNO
2
:
(a)
HNO
3
(b)
NaNO
2
(c)
NaOH
(d)
Mg(NO
2
)
2
(e)
KC
2
H
3
O
2
You Try It:
Buffer Capacity
o
DP to [ ]
o
DP to Volume
Buffer Effectiveness
o
Ratio of [A]:[B]
o
Most effective near 1
o
Acceptable 0.1-10
o
pKa = +/- 1 of desired pH
Buffer Math
Review Ch. 13
1.
pH HF
2.
Initial pH of Buffer
3.
pH after addition of base
4.
pH after addition of acid
1.
Write eqn
2.
ICE chart
3.
Choose x
4.
Solve
5.
Calc. pH
pH of pure HF Solution:
0.1 M HF K
a
(HF) = 7.1 x 10
-5
pH Initial Solution:
0.1 M HF + 0.1 M NaF K
a
(HF) = 7.1 x 10
-5
pH after addition of Base:
0.1 M HF + 0.1 M NaF K
a
(HF) = 7.1 x 10
-5
Add 0.01 M NaOH
Compare to 0.01 M
NaOH alone
pH after addition of Acid:
0.1 M HF + 0.1 M NaF K
a
(HF) = 7.1 x 10
-5
Add 0.01 M HCl
Compare to 0.01 M
HCl alone
Summary:
Buffer vs No Buffer:
o
Calculate the [HCl] required to have the same pH as a 0.1 M HF + 0.1 M (4.15)
o
Calculate the pH after addition of 0.01 M NaOH
o
Compare the Buffered system and Non-buffered system
Henderson-Hasselbalch Equation
“Buffer Shortcut”
Double Check!
[Acid] > [Base] then pH < pK
a
[Acid] = [Base] then pH = pK
a
[Acid] < [Base] then pH > pK
a
Example:
What is the pH of a buffer made from 0.2 M NH
3
and 0.3 M NH
4
Cl?
From Appendix
K
b
= 1.8 x 10
-5
Example:
Prepare a buffer with pH = 4.0
From Appendix
Example:
Prepare a buffer with pH = 9.0
From Appendix
Important Buffer Systems
Not on exam
Ocean Acidification
Not on exam
Fundamentals of buffer solutions including definitions, strengths, preparation methods, example systems, and mathematical calculations using the Henderson-Hasselbalch equation. Discover how buffers work, their capacity, and effectiveness through practical examples.
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Presentation Transcript
CHE 112 Spring 2021 Lecture 14 d Buffer Solutions Overview/Topics Skills to Master 1. Definition of Buffers 2. Buffer Strengths 3. Making Buffer Solutions 4. Examples of Buffer Systems 5. Buffer Math 6. Henderson-Hasselbalch Equation (HHE) 1. HW 14 d Read OER 14.5-14.6
Buffers o Solutions that are resistant to changes in pH o wa + salt of conjugate base o wb + salt of conjugate acid Salt = anion of SA cation of SB Weak Acid Salt of Weak Acid Replace H with Metal Ion
How Buffers Work Acid + Base = Salt + Water HC2H3O2 + NaOH NaC2H3O2 + H2O Salt CB + Acid = Weak Acid + Salt NaC2H3O2 + HCl HC2H3O2 + NaCl Equilibrium H3O+ (aq) + C2H3O2- (aq) HC2H3O2 (aq) + H2O (l)
You Try It: Which of the following is a Buffer Systems? (a) H2SO4 / Na2SO4 (b) HF / KF (c) NH3 / NH4Cl (d) NaOH / KOH (e) HC2H2O2 / HF You Try It: To make a buffer system you could add which of the following to HNO2: (a) HNO3 (b) NaNO2 (c) NaOH (d) Mg(NO2)2 (e) KC2H3O2
Buffer Capacity Buffer Effectiveness o DP to [ ] o DP to Volume o Ratio of [A]:[B] o Most effective near 1 o Acceptable 0.1-10 o pKa = +/- 1 of desired pH
Review Ch. 13 Buffer Math 1. Write eqn 2. ICE chart 3. Choose x 4. Solve 5. Calc. pH 1. pH HF 2. Initial pH of Buffer 3. pH after addition of base 4. pH after addition of acid 0.1 M HF Ka (HF) = 7.1 x 10-5 pH of pure HF Solution:
0.1 M HF + 0.1 M NaF Ka (HF) = 7.1 x 10-5 pH Initial Solution:
0.1 M HF + 0.1 M NaF Ka (HF) = 7.1 x 10-5 Add 0.01 M NaOH pH after addition of Base: Compare to 0.01 M NaOH alone
0.1 M HF + 0.1 M NaF Ka (HF) = 7.1 x 10-5 Add 0.01 M HCl pH after addition of Acid: Compare to 0.01 M HCl alone
Summary: Solution pH Pure 0.1 M HF: 2.07 Buffer 0.1 M HF and 0.1 M NaF 4.15 After adding Base: 4.23 After adding Acid: 4.06
Buffer vs No Buffer: o Calculate the [HCl] required to have the same pH as a 0.1 M HF + 0.1 M (4.15) o Calculate the pH after addition of 0.01 M NaOH o Compare the Buffered system and Non-buffered system
Henderson-Hasselbalch Equation Buffer Shortcut ?? = ???+ log[????] [????] Double Check! [Acid] > [Base] then pH < pKa [Acid] = [Base] then pH = pKa [Acid] < [Base] then pH > pKa
Example: What is the pH of a buffer made from 0.2 M NH3 and 0.3 M NH4Cl? From Appendix Kb = 1.8 x 10-5
Example: Prepare a buffer with pH = 4.0 From Appendix Compound pKa 3.77 Formic Acid Lactic Acid 3.85 Ascorbic Acid 4.10 Benzoic Acid 4.20
Example: Prepare a buffer with pH = 9.0 From Appendix
Important Buffer Systems Not on exam
Ocean Acidification Not on exam
