Titration of Vinegar

Introduction

Vinegar is a solution of a weak acid, acetic acid, in water. This acid will react with the base sodium hydroxide in a one-to-one molar ratio. If a solution of sodium hydroxide of known concentration is used to titrate a known volume of vinegar, it is possible to determine the amount of acetic acid that was present in the sample of vinegar.

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1. Identify specific hazards presented by the sodium hydroxide solution.
2. Identify precautions to take to prevent any injury due to the sodium hydroxide solution.
3. No precautionary statements about the vinegar are made in the lab procedure. Does this mean that the vinegar is a harmless solution? What hazards can the vinegar present and what precautions should be taken?
4. Using the Handbook of Chemistry and Physics (look under acid and base indicators in the index) describe how the indicator solution that you are using is prepared. The small quantity of phenolphthalein present in the solution is relatively harmless. One of the solvents is water. What is the other solvent and what are two precautions for handling the indicator solution safely?
5. When adding the vinegar to the wells, you are warned to make sure that all the drops fall directly to the bottom of the wells. Why is it important that the vinegar not run down the sides of the wells?
6. What is the appearance (color) of an acidic solution containing some phenolphthalein indicator?
7. What is the appearance (color) of a basic solution containing some phenolphthalein indicator?
8. When titrating a sample of vinegar by adding drops of the sodium hydroxide solution, how do you know when to stop adding the sodium hydroxide reagent?
9. If you use the same vinegar in each well and if each well is titrated with the same sodium hydroxide solution, why is it necessary to titrate three samples of the vinegar?
10. Write the balanced equation for the reaction of acetic acid solution with sodium hydroxide solution.
    

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1. Select a Beral pipet to use for the reagents. Fill the pipet with distilled water and then squeeze dry. Fill the pipet with a small portion of vinegar. Expel in the sink. Fill again with vinegar and expel in the sink. Fill the pipet with vinegar.
2. Select a 24-well plate for the experiment. Carefully transfer 50 drops of vinegar to each of 3 wells. Add 1 or 2 drops of phenolphthalein indicator to each of the 3 wells.
3. Squeeze the pipet dry. Rinse the pipet 3 times with small portions of distilled water. Discard the rinse at the sink. Fill the pipet with a small portion of sodium hydroxide base. Rinse. Expel. Repeat the rinsing procedure. Fill the pipet. (Instead, you may wish to use the two-piece device consisting of a plastic bulb and a pipet tip described above.)
4. Select one of the wells. Add base to this well, dropwise, with stirring, until a faint pink color is observed that remains for 30 seconds on standing. Record the number of drops required to reach this endpoint.
5. Repeat this procedure for the other 2 portions of vinegar.
6. Check your data. All three titration pairs should agree within 1 drop of one another. If they do not, repeat the titration.
7. Clean all your glassware and wash any left over acid or base down the sink.

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Prepare a data sheet to record the following information:

1. Trial number.
2. The amount of phenolphthalein indicator added to each well. Label this column accordingly.
3. The volume of vinegar added to each well.
4. The volume of sodium hydroxide solution added to each well to make the solution turn pink.

Record here the concentration, in moles per liter, of the sodium hydroxide solution from the label or provided by your teacher

[NaOH] =____________________

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Data Analysis

From the proportions in the balanced equation for the reaction between acetic acid and sodium hydroxide, complete the following mathematical relationship:

____ moles of acetic acid = ____ moles of sodium hydroxide

To determine the concentration of the acetic acid in each of the titrations, solve the following equation using the experimental data.

[CH₃COOH] = ([NaOH] x drops of NaOH)/drops of CH₃COOH

Calculate the concentration of the acetic acid in vinegar as determined by each of the three titrations you made. List your results here.

Average the values of the concentration of acetic acid in vinegar. Are any of the experimental concentrations very different from the average? If so, suggest some possibilities of experimental error to explain their dissimilarity. Compute the average without including widely discrepant data.

Assuming the density to be about the same as that of water, determine the weight percent of acetic acid in vinegar from the concentration determined above. Compare that value to the weight percent listed on the bottle.

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Safety

Wear eye protection at all times. Sodium hydroxide is corrosive to flesh and can cause blindness. Wash up spills immediately using large amounts of water. Have an eye wash available.

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1. The more serious hazard of sodium hydroxide is that it is caustic to flesh; it will cause blindness if it gets into the eye for just a few moments.
2. Goggles are worn at all times. Aprons and old clothing are worn. an eye wash is available.
3. Though it will cause pain and suffering, vinegar is not as dangerous as sodium hydroxide.
4. Phenolphthalein is dissolved in alcohol and then diluted with water.
5. If vinegar droplets hang-up on the sides of wells, they are counted as part of the added reactant but they are not available to react.
6. Acidic solutions of phenolphthalein are colorless.
7. Alkaline solutions of phenolphthalein are pink or red.
8. The base is added until the solution turns pink.
9. Repeat trials are performed to account for errors and mistakes. Errors are natural variations (e.g., small difference in drop size). Mistakes include forgetting to count a drop of base, or adding two when you think you only added one.
10. HOAc + NaOH --> NaOAc + H₂O

Typically concentrations of acetic acid run at 4.75 to 5.1 % acetic acid in vinegar.

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1. Theory of titration (if you have not done a titration).
2. Purpose of the indicator.
3. Formulas of acetic acid and sodium hydroxide.

Post-Lab Discussion:

1. pH of the reaction mixture during each step of the titration.
2. pH of the reaction mixture at the equivalence point.
3. Sudden change of color with the addition of one drop of base.
4. Possible errors which would make some concentration determinations out of line with the others.
5. Discuss the questions presented above.

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• Expected results - using solutions as described above, 50 drops of vinegar will require approximately 40 drops of sodium hydroxide
• For uniform drops, try the device described in the special technique section in which the stem of a thin stem plastic pipet is cut to 1" and inserted into a plastic pipet tip.

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• 5 mL commercial vinegar (leave label intact so students can read weight percent)
• 5 mL 1.0 M NaOH (20.0 grams of sodium hydroxide in a volumetric flask; add distilled water to a total volume of 500 mL)
• 1% phenolphthalein indicator (dissolve 1 g phenolphthalein in 60 mL of 95% ethanol; add enough distilled water to bring the total volume to 100 mL.)
• 24-well plate
• pulled Beral pipet

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The Woodrow Wilson microscale version of this lesson was developed by:

Mr. J. Timothy Perry
Mt. Hebron High School
9440 Route 99
Ellicott City, Maryland 21043

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