1. Unit 10 SUMMARY: Buffers & Acid-Base Titrations
weak conjugate acid-base pair.
resist pH changes (reacts added acid/base)
buffer capacity: more moles of buffer can react more acid/base before significant pH changes.
pH range: range of pH values buffer works.
Optimum Capacity and Range:
- weak acid with a pKa near desired pH
- [HA] = [A–] (so pKa = pH)

2. Common Ion Effect:
adding common ion shifts left causing the weak electrolyte to be less ionized.
pH of Buffers:
0.12 M lactic acid, HC3H5O (Ka = 1.4  10−4)
0.11 M sodium lactate, NaC3H5O3
HC3H5O3 + H2O  H3O+ + C3H5O3–
[H3O+] [C3H5O3−]
[HC3H5O3]
Ka =
I M 0 M M
C –x + x x
E – x x x
≈ 0.12 M ≈ 0.11 M
(x)(0.11)
(0.12)
1.4  10−4 =
x = 1.5  10−4 M H3O+
pH = –log(1.5  10−4) = 3.82

3. Titration
The analytical technique used to calculate the moles in an unknown soln.
mass % (g /gtotal)
molarity (mol/L)
molar mass (g/mol)
buret
titrant
end point:
indicator color change
known vol. (V)
known conc. (M)
equivalence point:
equal stoich. amounts (mols) react completely
Safari Montage (2 min) Titration
analyte
known vol. (V)
unknown conc. (M)
(or moles)

4. (irrelevant conjugate)
At Veq…
moles added =
moles reacted
…the solution contains only water & salt.
(irrelevant conjugate) SA
with SB
At Veq,
pH = 7

5. WA with SB At Veq, pH > 7 (only water & conj. base)
A– + H2O ↔ HA + OH–
Animation: (

6. WB
with SA
At Veq,
pH < 7
(only water & conj. acid)
HA + H2O ↔ H3O+ + A–
Indicators: weak acids with diff color conjugates
choose one that changes color (has pKa) near pH of equivalence point (Veq) of titration.

7. WA with SB Verbally describe the visual curve: Weak acids:
moderately low initial pH
gradual pH rise (not flat then jump)
subtle pH change at equiv. point (less steep)
Weak bases: (same
but “drop” not “rise”)

8. Titration Calculations
Calculate unknown moles of the ANALYTE.
(then calculate: M OR molar mass OR mass %)
Stoich: (L X  mol X  mol Y)
M X
Calculate unknown VOLUME of TITRANT (mL) added to reach equivalence (all reacted).
Stoich: (L X  mol X  mol Y  L Y)
M X M Y
Calculate unknown pH at any point in the titration (especially at equivalence).
Stoich: What’s in the flask? Find excess H+ , OH–