Determining the Percentage of Acetic Acid in Vinegar by Titration

I. Purpose
The purpose of this lab was to determine the percentage of acetic acid in vinegar by titration. We did this by adding sodium hydroxide to the vinegar until the acetic acid was neutralized and using an indicator to show this end point. We measured the amount of vinegar and sodium hydroxide used and converted the moles of sodium hydroxide used to moles of acetic acid. Then we divided the mass of acetic acid by the mass of vinegar to obtain the mass percentage of acetic acid.

II. Procedure
1. Fill one Beral pipet with vinegar and another with standardized NaOH. Weigh and record the mass of each.
2. Add 5 drops of vinegar solution to one well of a 24 well reaction plate or very small flask or beaker. Reweigh the vinegar pipet and record its new mass.
3. Add one drop of phenolphthalein to the well.
4. Add drops of NaOH solution to the well until one changes the vinegar solution to a pink color that does not disappear when mixed.
5. Reweigh the NaOH pipet and record its mass.
6. Repeat the procedure two more times. Carefully record the pipets’ mass before and after. If containers are reused, rinse out the previous titrants before each new procedure.

III. Observations
Adding phenolphthalein to vinegar produced no visible changes. Oftentimes, when NaOH was added, the solution turned pink; however, the color disappeared after the solution was stirred. Finally, a single drop of NaOH neutralized the acetic acid and permanently turned the solution pink.

IV. Equations and Chemical Formulas
Acid-Base Neutralization: NaOH + HC2H3O2 -> NaC2H3O2 + H2O
Mol = Volume * Molarity
Mol NaOH Used: Grams NaOH Used * Density NaOH * Molarity NaOH
Molarity of Vinegar: Moles HC2H3O2 / Liters of Vinegar Used
Mass of Vinegar Used: (Mass of Vinegar Pipet Before) – (Grams of Vinegar Pipet After)
Mass of NaOH Used: (Mass of NaOH Pipet Before) – (Mass of NaOH Pipet After)
Mol HC2H3O2: Mol NaOH
Grams HC2H3O2: Mol HC2H3O2 * Molar Mass HC2H3O2
% HC2H3O2 in Vinegar: Mass of HC2H3O2 / Mass of Vinegar * 100
Density NaOH: 1 g/mL
Density Vinegar: 1 g/mL

V. Data and Calculations
Category Trial 1 Trial 2 Trial 3
Grams of Vinegar Pipet Before 3.77g 3.62g 3.45g
Grams of Vinegar Pipet After 3.61g 3.46g 3.32g
Grams of NaOH Pipet Before 3.92g 3.22g 3.72g
Grams of NaOH Pipet After 2.54g 1.94g 2.56g

Category Trial 1 Trial 2 Trial 3
Grams of Vinegar Used 0.16g
(3.77g – 3.61g) 0.16g
(3.62g – 3.46g) 0.13g
(3.45g – 3.32g)
Volume of Vinegar Used 0.00016L
(0.16g * 1g/mL * 1L/1000mL 0.00016L
(0.16g * 1g/mL * 1L/1000mL) 0.00013L
(0.13g * 1g/mL * 1L/1000mL)
Grams of NaOH Used 1.38g
(3.92g – 2.54g) 1.28g
(3.22g – 1.94g) 1.16g
(3.72g – 2.56g)
Mol NaOH Used 0.000138 mol
(1.38g * 1g/mL * 1L/1000mL * 1mol/10L) 0.000128 mol
(1.28g * 1g/mL * 1L/1000mL * 1mol/10L) 0.000116 mol
(1.16g * 1g/mL * 1L/1000mL * 1mol/10L)
Mol HC2H3O2 0.000138 mol
1 mol HC2H3O2
1 mol NaOH 0.000128 mol
1 mol HC2H3O2
1 mol NaOH 0.000116 mol
1 mol HC2H3O2
1 mol NaOH
Grams HC2H3O2 0.00828g
(0.000138 mol * 60g/mol) 0.00768g
(0.000128 mol * 60g/mol) 0.00696g
(0.000116 mol * 60g/mol)
% HC2H3O2 in Vinegar 5.18%
(0.00828g HC2H3O2 / 0.16g Vinegar) 5.35%
(0.00768g HC2H3O2 / 0.13g Vinegar) 5.17%
(0.00696g HC2H3O2 / 0.15g Vinegar)
Average % HC2H3O2 in Vinegar 5.2%
5.18% + 5.35% + 5.17%
Molarity of Vinegar 0.86M
(0.000138 mol HC2H3O2 / 0.00016L vinegar used) 0.8M
(0.000128 mol HC2H3O2 / 0.00016L vinegar used) 0.89M
(0.000116 mol HC2H3O2 / 0.00013L vinegar used)
Average Molarity of Vinegar 0.85M
0.86M + 008M + 0.89 M

VI. Summary
The purpose of this experiment was to find the percentage of acetic acid in vinegar by titration. We titrated acetic acid and sodium hydroxide, using phenolphthalein as a catalyst, and determined the amount to be 5.1%.
This experiment was very simple and easy to perform, but since the measurements had to be so precise and the endpoint of the solution could be reached at any time, the experiment required the utmost precision. If our accuracy is ruined, the fault is mine because I added the drops of sodium hydroxide drops to the vinegar. Though I do not know the accuracy of our experiment, I believe that we were successful. We worked together and achieved our desired result. What more could be asked of three smashing young chemists?

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One Comment on “Determining the Percentage of Acetic Acid in Vinegar by Titration”

  1. Jerilene Says:

    Hi. i think you made a slight mistake in your summary. phenolphthalein is used as the indicator in the titration, not a catalyst.

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