Name______Block______Date______

Unit 4- Sec.IV-Lesson 22 – Titration Lab

Purpose

The purpose of this experiment is to determine the percent by mass of acetic acid in vinegar.

Introduction

Vinegar is a common ingredient used in cooking. Although there are many different types of vinegar (e.g. white vinegar, cider vinegar, red wine vinegar, etc.) all types of vinegar contain acetic acid (HC2H3O2). Indeed, in order to bear the name "vinegar", the solution must be 5% acetic acid in water by mass. Examination of the product label on any bottle of vinegar will show that the manufacturer states that the (acetic) acid content is 5%, by mass.

Molarity of a solution gives information on the concentration of the substance dissolved per liter of solution this makes. For example, 6 M (six molar) Hydrochloric acid has 6 moles of HCl per 1 liter of solution. This is very concentrated. 1M HCl is more dilute because there is only 1 mole of HCl per 1 liter of solution.

Titration is a common laboratory technique used to verify or determine concentration. Titration is a volumetric technique wherein a volume of a solution of known concentration (called a standard solution) is reacted with a solution of unknown concentration in order to establish the concentration of the unknown. In this experiment, standard sodium hydroxide, a strong base, will be used to assay vinegar for the concentration of acetic acid. Since the results from the titration are based on volume, some calculation will be required to establish the results of the assay as a mass percent.

Materials

  • 50 mL buret
  • ring stand with buret clamp
  • 10 mL graduated cylinder
  • 125 mL Erlenmeyer flask
  • two 100 mL beakers
  • ~70 mL 0.50 M NaOH
  • 1% phenolphthalein indicator in a dropper bottle
  • distilled water in a squirt bottle

Safety and Waste Management

Wear goggles and a lab apron. If any of the solutions get on the skin, flush with cold water immediately. Small quantities of vinegar or sodium hydroxide can be cleaned up with a paper towel. If any substantial amount of acid or base is spilled on counter, tell the instructor. Baking soda should be poured onto the spilled acid or base. (Baking soda neutralizes both acids and bases.) Then wash with water and dry with a paper towel.

The product from the titration is a salt solution of sodium acetate with trace quantities of phenolphthalein. All reaction mixtures may safely be disposed of down the drain. Use plenty of water to wash the equipment, and to flush the reaction products down the drain.

Pre-activity Questions Answer the following questions before starting the experiment.

  1. Describe what happens in a neutralization reaction?
  1. If you have 15 mL of 0.50 M NaOH, calculate the number of moles of NaOH.

Hint: M = mol/L 1000 mL = 1 L

  1. What color will the solution turn when the indicator shows you have neutralized the solution?
  1. If you have an accident at your laboratory bench and spill titrant (0.50 M NaOH) on the bench top, what is the best way to clean it up safely.

Procedure Be sure to record all data in the DATA TABLE

  1. Fill the buret with 0.50 M NaOH. Drain a small amount of the NaOH back into the beaker to ensure no bubbles are trapped inside of the buret tip.
  2. Add about 9 mL of vinegar into a graduated cylinder. Record the exact amount in the data table.
  3. Transfer the vinegar into a 125 mL Erlenmeyer flask. Use a wash bottle with distilled water to rinse any residual vinegar in the graduated cylinder into the flask. The addition of water to the flask does not affect the titration (why not?).
  4. Add 2 drops of phenolphthalein indicator to the Erlenmeyer flask.
  5. Record the initial volume of the buret in the data table. Beware! Burets are read from the top down.
  6. Carefully add NaOH from the buret into the flask containing the vinegar until a color change occurs. Add the titrant (0.50 M NaOH) drop-wise when the endpoint is near. The color change will be from colorless to pink. The first titration should be done quickly to get an idea of the volume needed to reach the endpoint.
  7. Record the final volume of the buret in the data table.
  8. Dispose of the pink solution in the flask down the drain (with the water running, of course!).
  9. Repeat the procedure. You may not need to refill the buret if enough NaOH remains to complete the trial. The graduated cylinder should be rinsed with vinegar to get rid of the water from the first transfer before measuring the second sample. The second trial should be done very carefully. The endpoint has been reached when a single drop of NaOH results in a permanent light pink solution.
  10. Repeat until you have two trials where the endpoint was reached with a single drop of NaOH. Titrate carefully!
  11. Dispose of any excess NaOH solution down the drain when you have completed your lab work. The NaOH solution should not be returned to the reagent bottle!

Data Table

1st Trial / 2nd Trial / 3rd Trial (if needed)
Volume of vinegar
Molarity of NaOH / 0.50 mol/L / 0.50 mol/L / 0.50 mol/L
Initial buret reading
Final buret reading
Volume of NaOH used

Calculations Table – Do calculations on the next page. Record final answers in this table.

1st Trial / 2nd Trial / 3rd Trial (if needed)
Moles NaOH
(answer to #2)
Moles acetic acid (answer to #3)
Mass of acetic acid (answer to #4)
Mass of vinegar (answer to #5)
Mass % of acetic acid in vinegar (answer to #6)

Calculations Show your work in the space provided. Record final answers in the table provided.

  1. In this experiment acetic acid reacts with sodium hydroxide:
  2. Write the word equation for the reaction.
  1. Write a balanced equation for the reaction.
  1. Determine the number of moles of NaOH used to neutralize the acetic acid in the vinegar in the two trials. (Hint: use the volume of NaOH in used in Liters and its molarity.) Show work and write the final answer in the table.
  1. According to the balanced equation, what is the mole ratio of acetic acid to sodium hydroxide?______How many moles of acetic acid were neutralized by the NaOH in each of the two trials?Show work and write the final answer in the table.
  1. Calculate the mass of acetic acid in the volume of vinegar used in each trial. (Hint: use your answer from #3 and the fact that the molar mass of acetic acid is 60 g/mol.) Show work and write the final answer in the table.
  1. Calculate the mass of the each vinegar sample from the volume of vinegar used. Assume density=1.0g/1.0 mL. Show work and write the final answer in the table.
  1. Calculate the mass percentage of acetic acid in vinegar for each trial
  1. Find the average mass percentage of acetic acid.
  1. As stated on the label on the bottle of vinegar, the accepted value for acetic acid in vinegar is 5% by mass. Determine the % error for the average mass % you obtained for your experiment.
    % error = (your experimental value − accepted value) x 100%
    (accepted value)