1. Experiment 9
Determination of Dissociation Equilibrium Constant of Acetic Acid
Purposes
1. Learn the method of determination
dissociation equilibrium constant.
2. Learn the method of using a pH meter to
determine the pH of solution.

2. Principles Acetic acid is a weak acid. There is dissociation
equilibrium in its solution.
Its acid dissociation constant Ka

3. Here, [H+], [Ac-] and [HAc] respectively are the
concentrations of H+, Ac-and HAc in equilibrium,
c is the initial concentration of HAc .
c can be determined by titration with NaOH standard solution, [H+] can be learned by determination the pH of HAc solution,
According to the equations listed above, Ka can be calculated.

4. Standardization of accurate/initial concentration of acetic acid
Procedure
Standardization of accurate/initial
concentration of acetic acid
Transfer 20.00mL HAc solution to a conical flask by
transfer pipette，add 2 drops phenolphthalein to the
solution. Use NaOH standard solution to titrated it until
the color of the solution turns pink and doesn’t disappear
in 30 seconds. Repeat this experiment three times.
Recoding and treating data. Calculate the concentration
of HAc solution
(cV)NaOH = (cV)HAc

5. 2. Preparation of HAc solutions with different concentrations
Transfer 20.00mL, 5.00mL and 2.50mL HAc solution
of accurately known concentration to three 50mL
volumetric flasks by transfer pipette or measuring
pipette respectively, then add distilled water to the
meniscus touch the mark in each volumetric flask
carefully and shake it to mix the solution.
Calculate the accurate concentrations of HAc
solutions respectively c1V1 = c2V2

6. 3. Measurement of the pH of four solutions above mentioned
Take out about 25mL of the above-mentioned
HAc solutions into four dry 50mL beakers
respectively. Determine the pH values of the
HAc solutions successively from dilute to
concentrated with a pH meter, and record the
pH value and the temperature. Calculate its Ka.

7. Table 1 Determine the concentration of HAc solution
4. Recording and treating data
Table 1 Determine the concentration of HAc solution
Date：
Temperature: ℃
Experiment No. 1 2 3
Indicator
Change of end point color
V(HAc)/ mL
Vfinal(NaOH)/ mL
Vinitial(NaOH)/ mL
⊿V(NaOH)/ mL
Vaverage(NaOH)/mL
c(NaOH)/mol·L-1
c(HAc)/mol·L-1

8. Table 2 The dissociation constant of HAc solution Date： Temperature: ℃
Experiment No 1 2 3 4
coriginal(HAc)/mol·L-1
Voriginal (HAc)/mL
2.50
5.00
20.00
50.00
VTotal (HAc diluted) /mL
c (HAc)/mol·L-1 pH
[H+] Ka
Ka (Average)
Table Ⅱ Determination of the content of acetic acid in vinegar