Purpose: To find the concentration of an acetic acid solution and to determine the acid's Ka from a titration curve.
Part I: Finding the Concentration of Acetic Acid
Using a burette, measure out into a small Erlenmeyer Flask approximately 10 mL of an acetic acid solution and record its volume to the nearest 0.05 mL. Dilute the acid to approximately 50 mL with distilled water and add 3 drops of phenolphthalein indicator to the resulting solution. Titrate the acid sample using a NaOH solution of known concentration as shown by your instructor. Repeat the titration an additional two times using a fresh acid solution for each trial. Make sure that you rinse your Erlenmeyer Flask between each trial.
Part II: Titration Curve
Measure approximately 5 mL of the same acetic solution into a rinsed Erlenmeyer Flask. Dilute the solution to approximately 100 mL with distilled water. Measure and record the pH of the solution using a pH meter as shown by your instructor. Add 1.0 mL of NaOH solution to the acid solution and swirl the flask to mix the solution well. Again measure the pH of the solution. Continue this procedure until you have added at least 5 mL of NaOH past the equivalence point (you will be able to tell you passed the equivalence point by noting the rapid rise in pH). NOTE: As you get closer to the equivalence point, use smaller increments than 1 mL (your goal is to NOT let the pH rise more than 1 pH unit in between additions). Be sure to record the exact total volume of NaOH added since the beginning of the titration at each point.
1. Determine the concentration of the acetic acid solution.
2. Make a graph of pH versus the volume of NaOH added to the acid solution. Draw a smooth curve through your data. Approximate the equivalence point of the titration as the steepest portion of your graph. Use your graph to determine Ka for acetic acid. Compare your value to the accepted value in the book.
Add the following questions to your discussion:
1. Does it make sense that the pH at the equivalence point for the titration was not 7? Explain.
2. Does the equivalence point as estimated from your graph correspond to the equivalence point you would predict from your calculations? Show calculations to support your answer.