This is the third lab in the series. In this lab students will use their standardized NaOH solution from lab 1 to determine the percentage by mass of acetic acid in vinegar (if you do not do lab 1, simply provide the students with a known concentration solution of NaOH).

            Students will place a known mass (approximately 5-8g) of an unknown vinegar solution into a 250mL Erlenmeyer flask, add indicator, and titrate with their NaOH solution from a burette. This will be repeated for a minimum of three trials OR until consistent results are obtained. I often “modify” the percentage of acetic acid by adding water or glacial acetic acid to the commercial vinegar sample. This keeps the students honest as they can not fudge their data to get the 5% that the bottle says.

• A bottle of commercial vinegar (altered or not as you choose)

For each group:

• Prepared 0.3 mol/L NaOH solution
• 25.00mL pipette (may be shared if needed)
• Pipette bulb
• 50.00mL burette
• 250mL Erlenmeyer flask
• Indicator (Phenolphthalein or phenol red)
• Wash Bottle with distilled-de-ionized water
• Glass stirring rod
• Balance (shared)

• Wear safety glasses at ALL times during the lab.
• Be aware of any cracked/chipped glassware.
• Be aware of any spilled acid as it can react with both skin and clothing.
• Wash hands after the lab.

• All products of the titration can be pored down the drain with running water.
• Be sure that the pipettes are rinsed with water before they are returned.
• Ensure that the students have rinsed their burettes with water, inverted them, and have left the stopcocks opened.

• Students will forget to add indicator to the acid in the flask.
• Students will fail to remove any air bubbles from the tip of the burette.
• Students will add the base too quickly near the end-point and overshoot the color change.
• Students will fail to use the appropriate number of significant figures when collecting their data.
• If the burettes are not properly rinsed and left with the stopcocks open, they will be plugged with solid NaOH by the start of the next day’s lab. This could take as much as 10-15 minutes to rectify.