This presentation explains how to determine the dissociation constant of an acid using pH meter.

1. INSTITUTE FOR EXCELLENCE IN
HIGHER EDUCATION
A presentation on Physical Chemistry
Submitted by-Rupal Agarwal
Roll No. 716058
Msc. Chem. Final year

2. Experiment
• Aim- To determine the pH of various mixtures of Sodium Acetate and acetic acid
and hence determine the dissociation constant of the acid.
• Materials Required- pH meter, beakers, buffer solutions, wash bottle, distilled
water, N/20 sodium acetate solutions, N/20 acetic acid.
• Preparation of solutions
• N/20 sodium acetate: weigh 0.82g of sodium acetate, transfer this into the
volumetric flask and make the volume up to 200ml using distilled water. The
solution prepared is N/20 sodium acetate.
• N/20 acetic acid: measure ml of N/ acetic acid and make the volume up to 200ml
using distilled water. This is the solution of N/20 acetic acid.
• Theory- The thermodynamic equilibrium constant of an acid Ha is given by
Ka = [aH3O+] * [a A-]
[aHA]
Or pH = pKa + [log aA+] = pKa + log[salt]
[aHA] [acid]

3. • A plot of pH against log salt/acid is approximately a linear graph. The
intercept on the axis of pH gives pKa.
• Acetic acid and acetate ions are conjugate acid-base pairs. A conjugate
acid is a substance that has one more proton in it’s structure than it’s
corresponding conjugate base. This combination also results form a
mixture of a weak acid, acetic acid, and it’s salt, sodium acetate. The
equilibrium constant expression is:
• Ka = [H3O+] [C2H3O2
-]
[HC2H3O2]
• If a solution contains equal concentrations of HC2H3O2 and C2H3O2
- these
concentration terms cancel out in the above equation so that ka= [H3o+].
• Procedure- Turn on the pH meter 30 minute before performing the
experiment.
• Prepare different buffer solutions of 4, 7, 9.2 to calibrate the apparatus.
• Now prepare various solutions of sodium acetate and acetic acid as
mentioned in the table.

4. • Measure the pH of each of the solution in turn.
• The plot of pH against log [sodium acetate]/ [acetic acid] and from the
intercept of the linear plot on the ordinate axis deduce pKa and hence Ka
of the acid.
• Observations and calculations:
Volume of acetic acid (ml) Volume of sodium acetate(ml)
5 35
10 30
15 25
20 20
25 15
30 10

5. S No. Acetic acid + Sod.
Acetate (ml)
pH Log [Salt]
[Acid]
Ka
1 5+35 3.790 0.845 1.135*10-3
2 10+30 3.379 0.477 1.253*10-3
3 15+25 3.130 0.221 1.233*10-3
4 20+20 2.799 0.000 1.588*10-3
5 25+15 2.649 -0.221 1.349*10-3
6 30+10 2.464 -0.477 1.146*10-3
7 35+10 2.144 -0.845 1.026*10-3

7. • Result: The dissociation constant of the given acid is 1.25 *10-3 .
• Precautions: Acids are harmful to skin and eyes so working should be done
carefully.