Objective:
To determine the molarity of acetic acid in a vinegar sample by titrating with a sodium hydroxide (NaOH) solution of known concentration.
Materials Needed
- Vinegar sample (acetic acid solution)
- NaOH solution (known concentration)
- Phenolphthalein indicator
- Burette
- Pipette
- Erlenmeyer flask
- Distilled water
Procedure
- Preparation:
- If necessary, dilute the vinegar sample to ensure an appropriate titration volume.
- Measure a fixed volume of the vinegar sample (e.g., 10 mL) and transfer it to an Erlenmeyer flask.
- Adding the Indicator:
- Add a few drops of phenolphthalein indicator to the vinegar sample. The solution should remain colorless initially, as acetic acid is a weak acid.
- Titration:
- Set up the burette with the NaOH solution.
- Begin titrating by slowly adding NaOH to the vinegar solution while continuously swirling the flask.
- Stop adding NaOH once a pale pink color appears and persists for at least 30 seconds. This stable pink color indicates the endpoint.
- Calculation:
- Use the volume of NaOH added to calculate the moles of NaOH used, which is equal to the moles of acetic acid in the vinegar sample.
Calculation of Molarity of Acetic Acid
Molarity of Acetic Acid (M) = Moles of Acetic Acid Volume of Vinegar Sample in Liters
Results Table Example
| Volume of Vinegar (mL) | Concentration of NaOH (M) | Volume of NaOH Used (mL) | Moles of NaOH (mol) | Moles of Acetic Acid (mol) | Molarity of Acetic Acid (M) |
|---|---|---|---|---|---|
| 10 | 0.10 | 25 | 0.0025 | 0.0025 | 0.25 |
In this example, a 10 mL sample of vinegar required 25 mL of 0.10 M NaOH to reach the endpoint. Calculations based on this setup yield the molarity of acetic acid in the vinegar.
FAQs
Q1: Why is phenolphthalein used as an indicator?
A1: Phenolphthalein is a suitable indicator for acid-base titrations because it changes color at a pH range of about 8.2–10.0. In this experiment, it provides a visible indication of the endpoint when acetic acid is neutralized by NaOH.
Q2: How do I know when the endpoint has been reached?
A2: The endpoint is reached when the solution in the Erlenmeyer flask turns a pale pink color that lasts for at least 30 seconds.
Q3: What does the result tell me about the vinegar sample?
A3: The result provides the molarity of acetic acid in the vinegar. This information can be used to determine the strength or acidity of the vinegar sample.
Q4: Can other indicators be used in this experiment?
A4: Yes, but the chosen indicator should have a color change in a range that is suitable for detecting the neutralization point in a weak acid-strong base titration. Phenolphthalein is preferred because it changes color near the equivalence point in this type of titration.
Q5: What is the importance of diluting the vinegar sample?
A5: Dilution may be necessary to control the amount of NaOH required to reach the endpoint, ensuring the experiment remains practical and manageable with standard lab equipment.
This comprehensive guide, with results in a table and clear answers to common questions, should provide students with a solid understanding of the procedure and calculations required in titrating acetic acid with NaOH.
