Titration Analysis of Vinegar

Vinegar is conveniently analyzed by titration with sodium hydroxide. 

Problem

What is the concentration of acetic acid, CH3COOH(aq) in a sample of vinegar?

Experimental Design

Several 25.00 mL samples of NaOH(aq) are titrated with an unknown solution  of acetic acid. Bromothymol blue , is used to detect the equivalence point. The indicator changes from blue to pale yellow at the endpoint of the reaction.

Watch a titration video to learn proper lab techniques.

Evidence

Flash object created by Chemical Education Research Group, Department of Chemistry, Iowa State University

Procedure

1. Follow the numbered steps. First select weak acid strong base.
2. Fill the buret with Acid.
3. Select the acid (CH3COOH) and the base (NaOH).
4. Select the indicator (Bromothymol blue).
5. Record the volume and molarity of the base in the flask.
6. Click on the Concordant Values button to fill the buret.
7. Push the slider up to add a volume of acid that will cause a color change.
8. Make note of the point at which the color change occurs and refill the buret for another trial.
9. This time move the slider to a point slightly less than the previous step.
10. Use the Dropwise button beside the buret to add titrant to the sample solution.
11. When a drop of titrant changes the color of the sample to pale yellow take a final buret reading.
12. After titration calculate the molarity of the base and enter it in step 6. Click OK to see if your calculation is correct.
13. Repeat the process three more times.
14. Compare the results of each trial.

Evidence Table:

Trial 1 2 3 4
Molarity of NaOH(aq)        
Volume of NaOH(aq) 25mL 25mL 25mL 25mL
Volume of CH3COOH(aq) added        
Molarity of CH3COOH(aq)        

To calculate the molarity of the acid multiply the molarity of the base by the volume (25mL) and divide by the volume of the acid added.

For each of your trials, show your calculations for determining the concentration of the vinegar.

Evaluation:

1. When performing a titration, it is important not to add the titrant too quickly.  Explain why adding titrant too quickly would give poor results.
2. A buret is a device designed to measure the volume of a solution precisely.  Expressed in litres (L) what is the volume of a single drop using the buret in this applet?
3. Bromothymol blue is an acid-base indicator that changes colour between pH 6.0 7.6.  What is the purpose of the bromothymol blue in this lab?
4. What do we know about the hydroxide ions (OH-(aq)) in solution when we reach the endpoint?
5. A good student would wash the Erlenmeyer flask with water before adding the barium hydroxide for the next trial.  If the student accidentally left 5.0 mL of distilled water in the flask before adding the 25 mL of barium hydroxide, what effect would this have on the results of the analysis?
6. A good student would wash the Erlenmeyer flask with water before adding the barium hydroxide for the next trial.  If the student accidentally left 5.0 mL of distilled water in the flask before adding the 25 mL of barium hydroxide, what effect would this have on the results of the analysis?
7. If water is left in the buret after washing, and then the acid is added to the buret, would the analysis of the concentration of vinegar be affected?
8. In this lab, the solution with the unknown concentration was placed in the buret.  Explain how you could determine the concentration of the acid if the barium hydroxide had been placed in the buret.
   

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