Ch 16: Acid-base equilibria
•What’s the pH of 0.200M acetic acid?
K
•What’s the pH of a solution that is made of 0.200M acetic acid and 0.100M sodium acetate? Remember that salts are 100% dissociated into their ions.•pH=•Le Chatelier: in the presence of a common ion(A-), the•What is the pH of 1.50M pyridine, C5H5N?
-
A-
•What is the pH of a solution that is 1.50M in pyridine and 1.500M in pyridinium chloride, C5H5NHCl?
-
•What is the pH of a 1.0 x 10-8M sol’n of HCl?

.
and
1
10
•How do buffers work?
HA
+
A-
+
•Have 100mL of 3.6x10-5M HCl; pH = 4.44
•What if had a buffer sol’n--what happens when add acid and base to the buffer soln?
•Practice exercise: Calc the pH of the 0.30 M NH3/ 0.36 M NH4Cl buffer. What is the pH after the addition of 20.0 mL of 0.050 M NaOH to 80.0 mL of the buffer?
•What is the pH after the addition of 20.0 mL of 0.050 M HCl to 80.0 mL of the buffer?

•
16.18
Calc the pH of 1.00 L of the
buffer 1.00 M CH
3
COONa/1.00 m
CH
3
COOH. What is the pH after add
0.08 mol NaOH
•
Are these buffers:
•
KOH/KCl
•
NaOH/HCl
•
NH
3
/NH
4
Cl
•
NaF/HF
•
Na
2
HPO4/NaH
2
PO4
•
NaOH/HF
•
NH
3
/HCl
•
HF/HCN
•
CO
2
/ HCO
3
-
•
What mass of C
6
H
5
COONa (sodium
benzoate) do you need to add to 400.
mL of 0.500M C
6
H
5
COOH (benzoic
acid) to make a buffer of pH=4.540? K
a
= 6.5x10
-5
for the acid.
•
16.15: The pH of a sodium acetate-
acetic acid buffer is 4.50. Calculate the
ratio [CH
3
COO-]/[CH
3
COOH].
Henderson-Hasselbalch
•
HA
H
+
+ A-
K
a
= [H
+
][A-]/[HA]
•
take log of both sides
logK
a
= log[H
+
] + log{[A-]/[HA]}
•
multiply thru by -1 and rearrange
•
pH =pK
a
+ log{[A-]/[HA]} or
•
pH = pK
a
+ log {[conj base]/[acid]}
•
pK
a
+ pK
b
= 14
or K
a
K
b
= 1x10
-14
for any conj
acid/base pair
•
For buffers ratio
of [conj base]/[acid]
btn 0.1 and 10
•
So pH = pK
a
+ log {[conj base]/[acid]} or
•
pH = pK
a
good for preparing a
buffer of a desired pH.
