Vinegar_titration

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Chemistry

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Jan 9, 2024

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1 Titration of Vinegar Introduction: Titrations are procedures for determining the concentration of an unknown acid or base using a known (standard) solution of either acid or base. For example, if a student wanted to know the concentration of unknown acid they would titrate with a known concentration of base. Alternatively, if a student wanted to know the concentration of an unknown base, they would titrate with a known concentration of acid. In today’s laboratory, we will determine the concentration of acetic acid (CH 3 COOH) in a vinegar sample. The concentration of CH 3 COOH in a vinegar sample can be determined by reacting that sample with a solution containing a known concentration of sodium hydroxide (NaOH), which is a base. Acetic acid reacts with NaOH by the chemical equation shown below: CH 3 COOH + NaOH → NaCH 3 COO + H 2 O This reaction is a neutralization reaction, which reacts an acid (acetic acid) with a base (sodium hydroxide) to produce a salt (sodium acetate) and water. To begin the titration, we will measure out a known volume of vinegar and then add an acid/base indicator, phenolphthalein. The phenolphthalein will change color when the neutralization reaction is complete. We will fill a calibrated glass tube called a buret with a known concentration of sodium hydroxide. Gradually, we will add sodium hydroxide to the vinegar sample until the neutralization reaction is complete as indicated when the solution turns a pale pink. Reading from the buret gives the volume of the NaOH solution that reacted with the CH 3 COOH. Knowing the concentration (Molarity) of NaOH and the volume of NaOH added, you can calculate the number of moles of NaOH. Using the balanced chemical equation above, you can compare the number of moles of NaOH to the number of moles of CH 3 COOH and obtain the number of moles of CH 3 COOH. However, you are trying to determine the concentration of CH 3 COOH. The concentration is defined by Molarity (moles of CH 3 COOH/ L of CH 3 COOH). You have calculated the moles and you know the volume of CH 3 COOH (your volume of vinegar). The concentration of the acetic acid can be calculated. Shown below is an example problem. When a 5.00 mL sample of vinegar is titrated, 44.5 mL of 0.100 M NaOH is required to neutralize the reaction. The chemical equation is: CH 3 COOH + NaOH → NaCH 3 COO + H 2 O (44.5 mL NaOH) ( 1 Liter NaOH 1000 mL NaOH )( 0.100 mole NaOH 1 L NaOH )( 1 mole CH 3 COOH 1 mole NaOH )( 1000 mL CH 3 COOH 1 L CH 3 COOH )( 1 5.00 mL CH 3 COOH ) = 0.890 M CH 3 COOH

2 Procedure: Supply List (Items you will need to gather before starting the procedure) 125-mL Erlenmeyer flask 100-mL graduated cylinder 250-mL Beaker Buret with stand Funnel Preparations: Vinegar sample 1. Wash the 125-mL Erlenmeyer flask with soap and rinse with tap water followed by rinsing with distilled water. 2. Pipet 10-mL of vinegar into the Erlenmeyer flask. Your laboratory instructor will help you pipet the vinegar. Record the volume of vinegar you transferred on to your data sheet 3. Using your graduated cylinder add 40 mL of distilled water to the vinegar sample in the flask. 4. Add 3 drops of phenolphthalein to the vinegar solution and gently swirl to mix. 5. Label your vinegar sample with a Sharpie and set to the side. NaOH standard solution 1. Using a graduated cylinder, obtain 60 mL of NaOH solution. On your data sheet record the molarity of the NaOH (it is on the label of the NaOH bottle). 2. Make sure the stopcock is closed on your buret. If you are unsure ask your instructor to check. 3. Place a 250-mL beaker directly under the buret tip. Lower the buret so that the tip of buret extends about 1-2 inches into the beaker 4. Place a funnel in the open top of buret. 5. Fill the buret with NaOH to a level above the 0-mL calibration line at the top of the buret. As the NaOH solution approaches the top of the buret, pour very slowly. Add only enough NaOH to bring the volume in the buret to slightly over 50 mL. CAUTION: Adding more NaOH than the buret can hold will cause NaOH to overfill the buret and cause a chemical spill! NaOH is corrosive and toxic. 6. Slowly drain the NaOH solution from your buret into the beaker until the bottom of the meniscus is at the 0-mL calibration. Read the meniscus to the nearest 0.1 mL and record this initial reading on your data sheet. 7. Remove the beaker from the under the buret.

3 First Titration 1. Place your Erlenmeyer flask containing your vinegar under the buret tip. Lower the buret so that the tip extends into the flask. 2. Slowly, add the NaOH solution from the buret to the vinegar while gently swirling. 3. Stop adding NaOH from the buret when a light pink persists throughout the vinegar solution for 30 seconds after you have thoroughly swirled the flask. 4. After the titration, take a final buret reading and record this reading to the nearest 0.1 mL on your data sheet. 5. Discard your vinegar sample in the sink. Second titration 1. You will need to prepare a second vinegar sample: a. Wash each 125-mL Erlenmeyer flask with soap and rinse with tap water followed by rinsing with distilled water. b. Pipet 10-mL of vinegar into the Erlenmeyer flask. Your laboratory instructor will help you pipet the vinegar. Record the volume of vinegar you transferred on to your data sheet. c. Using your graduated cylinder add 40 mL of distilled water to the vinegar sample in the flask. d. Add 3 drops of phenolphthalein to the vinegar solution and gently swirl to mix. e. Place your Erlenmeyer flask containing your vinegar under the buret tip. Lower the buret so that the tip extends into the flask. 2. Record the amount of NaOH solution remaining in your buret. This volume should be recorded on your data sheet as the initial volume for the second titration. 3. Slowly, add the NaOH solution from the buret to the vinegar while gently swirling. 4. Stop adding NaOH from the buret when a light pink persists throughout the vinegar solution for 30 seconds after you have thoroughly swirled the flask. 5. After the second titration, take a final buret reading and record this reading to the nearest 0.1 mL on your data sheet. 6. Discard your vinegar sample in the sink. Waste Disposal: Discard the vinegar samples in the sink. Your laboratory instructor will tell you how to dispose of the NaOH in the buret.

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