Acid / Base Titration – Ka (HAc)
• Hand in: Pre-Lab and Equilibrium report and Submit Group Form
• OPEN EXCEL
• Today - Weak Acid: HA(aq)
H+(aq) + A-(aq)
 
[ H ][ A ]
Ka 
[ HA]
Determine Ka by performing a titration:
CH3COOH(aq) + NaOH (aq)
about 0.1M
NaCH3COO(aq) + H2O (l)
0.0986 M in 2011
We can use the 1:1 molar relationship and titration endpoint to determine
exact M of Acetic Acid (like prelab question #2) – how do we know the
endpoint?
WE MAKE A TITRATION AND DERIVATIVE CURVE
Acid / Base Titration – Ka (HAc)
• Set up EXCEL with active titration graph
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Column A – mLs of NaOH added – sig figs dictated by buret
Column B – pH (standard units) of solution
Column C – Derivative (ΔpH/Δ Volume) or (B3-B2/A3-A2)
Set up for real time graph (pH vs. volume added)
• Set up Titration Apparatus
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Stirring Plate, Ring Stand, Clamp (for pH probe), Burette Holder, Burette
Rinse Burette with 0.1 M NaOH, no bubbles, Fill with NaOH to 0.00 mL
Add 25.00 mL of Acetic Acid to clean, dry beaker (use volumetric pipet)
Add 50.0 mL of DI water using grad cylinder
Add 4-5 drops of Phenolphthalein
Add stir bar
Acid / Base Titration – Ka (HAc)
Perform Titration
Calibrate the pH probe
use manual instructions
use pH 7.00, then pH 4.00
Always rinse probe with DI water, then dab dry gently
BE PATIENT WHEN WAITING FOR FLASHING NUMBER
TO STOP!!!
Begin Titration
Add 1 mL increments until pink color fades slowly
Switch to 0.10 to 0.20 mL increments
Record EXACT volumes and stabilized pH values
As you get closer to endpoint, derivative value will jump
continue past endpoint until your values stabilize (above pH 10)
“Save” (and save often) your file(s) – perform titration 3x’s
Ka Calculation: HA
Conc’ns
Initial
¼-equiv
½-equiv
¾-equiv
[HA]
1M
¾M
½M
¼M
[A-]
H+ + AKa 
[H
 ][ A ]
[ HA]
0M
¼M
[ H  ](1 / 4)
(3 / 4)
[H  ]
Ka 
3
½M
[ H  ](1 / 2)
(1 / 2)
Ka  [H  ]
¾M
[ H  ](3 / 4)
(1 / 4)
K a  3 [H  ]
pH   log[ H  ] or [ H  ]  10 pH
Decimals in pH = Sig Figs in [H+]
Acid / Base Titration – Ka (HAc)
Titration and Derivative Curves,
NaOH + Acetic Acid
Titration Curve
Derivative
14.00
25.00
Highest point in derivative curve
indicates endpoint volume, 23.50 mL
12.00
10.00
3/4 endpoint, 17.63,
1/2 endpoint, (11.75, 5.24
4.76)
pH
8.00
6.00
15.00
1/4 endpoint, (5.88,
4.28)
10.00
4.00
5.00
2.00
0.00
0.00
5.00
10.00
15.00
Volume of .103 M NaOH (mLs)
20.00
25.00
0.00
30.00
Derivative (ΔpH/Δ Volume)
20.00
Lab Report
 Title Page
 Purpose
 Results
- Three Titration/Derivative Graphs (don’t need data columns)
one with ¼, ½, and ¾, and endpoint clearly marked
endpoint clearly marked on other two
- Data Sheet (with sample calculations)
- Present/Staple in order data/calculations were obtained/performed
- Significant Figures and Units
 Conclusion [ Errors , Improvements ]
 Individual Checked-Pre-labs
Lab Report Due Next Week