Chem. 31 – 5/4 Lecture
Announcements
• Lab Stuff
– Draft Formal Lab Report (for peer review) due Mon.
+ Tues. (today and tomorrow)
– Peer Review due Wed. + Thurs. in lab
– IC and Formal Lab Reports due 5/11
– Check out on 5/13, 5/14
• Today’s Lecture
– Chapter 9: Polyprotic Acids
• Fraction of acids IN buffered solutions
– Chapter 10: Acid – Base Titrations
• Strong Acid – Strong Base Titrations
Chapter 9 – Polyprotic Acids
• Distribution of forms at fixed pH (e.g. when in a
buffer)
• Let’s start with monoprotic acid HA
– HA H+ + A– Fraction present as HA = a(HA) = [HA]/[HA]total =
[HA]/([HA] + [A-])
– Can calculate ratio ([A-]/[HA]) using HendersonHasselbalch equation, but other method calculating a
values is more general
– Go over method on board
 a(A-) = Ka/([H+] + Ka) (can do derivation yourself)
Chapter 9 – Polyprotic Acids
• On to a diprotic acid
H2A
aH
2A
[ H  ]2

[ H  ]2  K a1[ H  ]  K a1 K a 2
From H2A form
From HA- form
From A2- form
a HA

K a1[ H  ]

[ H  ]2  K a1[ H  ]  K a1 K a 2
Can you guess a(A2-)?
Notice that denominator is
the same
Chapter 9 – Polyprotic Acids
Example Problems
• Calculate the pH of 0.080 M
NaNH2CH(CH2OH)CO2 (an amino acid:
pKa1 = 2.187, pKa2 = 9.209)
• What is the concentration of CO32- if 1.0 x
10-4 moles of CO2 are dissolved into 1.0 L
of a buffer at pH = 8.80? (for H2CO3, pKa1
= 6.35, pKa2 = 10.33.)
Chapter 10 – Acid Base Titrations
• Main Chapter Goal: Covering shape of titration
curves including pH vs. V calculation
• Why is titration curve shape important?
– Affects accuracy and precision of titration
– Affects selection of indicator to use
• Possible Titrations:
–
–
–
–
Strong Acid – Strong Base Titrations
Monoprotic Weak Acid – Strong Base Titrations
Monoprotic Weak Base – Strong Acid Titrations
Other Titrations: Polyprotic Acids, mixtures of acids
Chapter 10 – Acid Base Titrations
Strong Acid – Strong Base Titrations
• Strong Acid – Strong
Base Titration
– How does pH Change as
NaOH is added?
– 3 regions to titrations
(different calculations in
each region):
• before equivalence point
• at equivalence point
• after equivalence point
– Go to Board to show pH at
5 mL, 12.5 mL, and 15 mL
0.100 M NaOH
0.050 M HCl, 25 mL
Chapter 10 – Acid Base Titrations
Strong Acid – Strong Base Titrations
Titration Plot
0.100 M NaOH
Titration Plot
14.00
12.00
pH
10.00
8.00
0.050 M HCl, 25 mL
6.00
4.00
2.00
0.00
0
5
10
15
V(HCl)
20
25
Chapter 10 – Acid Base Titrations
Strong Acid – Strong Base Titrations
• What affects shape of
curve?
Titration Plot
14.00
12.00
10.00
pH
– Concentration of acid and
base
– Curves are normally sharp.
Only exception occurs
when intial concentrations
are very low (e.g. [H+]o,
[OH-]o < 10-4 M)
– Note: At low
concentrations, one also
needs to ensure CO2
doesn’t affect NaOH
solutions
50 mM acid
8.00
1 mM acid
6.00
50 uM acid
4.00
2.00
0.00
0
10
20
30
V(HCl)
Note: Base conc. changed by same
factor
Chapter 10 – Acid Base Titrations
General Acid – Base Titrations
• What affects accuracy and precision?
– Depends on method for determining equivalence point
– End point = “measured” volume of titrant
– Titration error (systematic) = difference between end
point and equivalence point
– With indicators, systematic errors occur when pH(end
point) ≠ pKa(indicator) – This causes the titration error
– With indicators, random errors occur from difficulty in
determining correct color shade which depends on
[In-]/[HIn]
– A factor of 2 uncertainty in [In-]/[HIn] (e.g. 0.5 to 2)
translates to pHmeas = pHtrue + 0.3
Chapter 10 – Acid Base Titrations
General Acid – Base Titrations
• What affects accuracy and precision?
Example 1 – precise + accurate titration
Uncertainty from
judging indicator
color (e.g. Methyl
orange)
pH
Vol. Base
sV is small
Chapter 10 – Acid Base Titrations
General Acid – Base Titrations
• What affects accuracy and precision?
Example 2 – mainly imprecise – due to
unsharp titration (e.g. low concentrations)
Uncertainty from
judging indicator
color (e.g. Methyl
orange)
pH
Vol. Base
sV is large
Chapter 10 – Acid Base Titrations
General Acid – Base Titrations
• What affects accuracy and precision?
Example 3 – mainly inaccurate titration
(due to selection of wrong indicator
equiv. pt. pH
pH
indicator color change
region (pH < pKa)
Vol. Base
small unc. in V but significant error