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.

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

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.

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

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

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.

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

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-18.3.3 Determination of the content of acetic acid in vinegar