Experiment 9 Determination of Dissociation Equilibrium Constant of

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 p. H meter to determine the p. H 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 Na. OH standard solution, [H+] can be learned by determination the p. H of HAc solution, According to the equations listed above, Ka can be calculated.

Procedure 1. Standardization of accurate/initial 2. concentration of acetic acid Transfer 20. 00 m. L HAc solution to a conical flask by transfer pipette，add 2 drops phenolphthalein to the solution. Use Na. OH 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 (c. V)Na. OH = (c. V)HAc

2. Preparation of HAc solutions with different concentrations Transfer 20. 00 m. L, 5. 00 m. L and 2. 50 m. L HAc solution of accurately known concentration to three 50 m. L 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. c 1 V 1 = c 2 V 2

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

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

solution Table 2 The dissociation constant of HAc solution Date： Experiment No coriginal(HAc)/mol·L-1 Voriginal (HAc)/m. L VTotal (HAc diluted) /m. L c (HAc)/mol·L-1 Temperature: ℃ 1 2 3 4 2. 50 5. 00 20. 00 50. 00 p. H [H+] Ka Ka (Average) Table Ⅱ-18. 3. 3 Determination of the content of acetic acid in vinegar