CHEM 1312 Lecture Notes - Lecture 9: Standard Solution, Titration Curve, Partition Coefficient
Chapter 17: Acid-Base Equilibria
and Solubility Equilibria
Focus of this section: Buffers
Acid-base titrations
Solubility equilibria
Common Ion Effect:
• until now – mainly dealt with single solute in solution
• now deal with 2 solutes
• when add ion that is in common with already dissolved
species, you shift the equilibrium
= common ion effect
• Example: 1.0 L of 0.10 M solution of CH3COOH
-- add 0.050 mol of CH3COONa shift to left
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Minus common ion:
Determine pH of 1.0 L of 0.10 M solution of CH3COOH
M
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Plus common ion:
M
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
5
a1.8x10−
=K
0.10
0.10 − x
− x
+ x
x
x
+ x
0
0
5
22
3
3
a10 x 1.8
0.10x 0.10COOH][CH
]][HCOO[CH −
+−
=≈
−
== xx
K
3
10 x 1.34][H
−+
=
2.871.34x10pH 3=−=−)log(
5
a10 x 1.8 −
=K
0.10
0.10 − x
− x
+ x
x
+ x
0.050 + x
0.050
0
5
3
3
a10 x 1.8
0.10
) )(0.050(
0.10
) )(0.050(
COOH][CH
]][HCOO[CH −
+−
=≈
−
+
== x
x
xx
K
5
10 x 3.6][H
−+
=
4.4410 x 63pH 5=−=−).log(
4.442.87 <
• presence of common ion shifts equilibrium to left
-- suppresses ionization of weak electrolyte (acid)
• important points:
1. ignored water autoionization
2. will reach same point if we:
-- add acetate to acetic acid solution
-- add acetic acid to acetate solution
-- add acetic acid and acetate to water
3. could have begun “Plus common ion” problem with
equilibrium concentrations from first problem:
y = 1.304 x 10–3 M
[H+] = 3.6 x 10–5 M pH = 4.44 (same value!)
• simplest to assume CH3COOH and CH3COO– added at
same time
Buffer Solutions: no new concepts—just new application
• Buffers important:
-- pH of blood must be maintained between 7.38 and 7.42
-- if varies outside of that YOU DIE
H2CO3 / HCO3
– / CO3
2– / CO2
5
a10 x 1.8 −
=K
0.09866
0.09866+y
+y
–y
1.34x10–3 –y
–y
5.134x10–2 –y
5.134 x 10–2
1.34 x 10–3
Document Summary
Common ion effect: until now mainly dealt with single solute in solution, now deal with 2 solutes, when add ion that is in common with already dissolved species, you shift the equilibrium. = common ion effect: example: 1. 0 l of 0. 10 m solution of ch3cooh. - add 0. 050 mol of ch3coona shift to left. Determine ph of 1. 0 l of 0. 10 m solution of ch3cooh. 5: presence of common ion shifts equilibrium to left, important points: - suppresses ionization of weak electrolyte (acid: ignored water autoionization, will reach same point if we: - add acetic acid and acetate to water: could have begun plus common ion problem with equilibrium concentrations from first problem: 5. 134x10 2 y y = 1. 304 x 10 3 m. [h+] = 3. 6 x 10 5 m: simplest to assume ch3cooh and ch3coo added at ph = 4. 44 (same value!) same time. Buffer solutions: buffers important: no new concepts just new application.
