chem ch 16 - Ch 16 Acid-base equilibria Whats the pH of...

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,
C
5
H
5
N?
-
A-
• What is the pH of a solution that is
1.50M in pyridine and 1.500M in
pyridinium chloride, C
5
H
5
NHCl?
-
• What is the pH of a 1.0 x 10
-8
M sol’n of
HCl?

.
and 1
10
• How do buffers work?
HA
+ A-
+
• Have 100mL of 3.6x10 -5 M 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 NH
3
/ 0.36 M NH
4
Cl 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.