% ACETIC ACID IN VINEGAR

(Titration of Vinegar with a handheld pH meter)

INTRODUCTION:

In the field of analytical Chemistry, one of the most widely used techniques in quantitizing an unknown acid or base is known as titration. For this experiment we will titrate vinegar with sodium hydroxide (NaOH) utilizing a pH meter. This technique is based upon pH (power of the hydronium ion in [H3O+]) or pOH (power of the hydroxide ion in [OH-]). These two ions come into play when an acid or base are reacted with water. When an acid reacts with water, hydronium ions are produced and the pH of that reaction can be determined. Similarly, when a base reacts with water, hydroxide ion is produced. When an acid is titrated with a base, a certain amount of base is required to bring the reaction to equivalence or (neutralization point). The equivalence reaction is:

H3O+(aq) + OH-(aq) 2H2O(l)

Using this information (along with stoichmetric equations) one can determine the amount of acid present in a solution.

PRE-LAB QUESTIONS:

1. What does pH stand for?

2. What does pOH stand for?

3. What is the definition of "neutralization"?

4. Given the structural formula for acetic acid (H C C OH), What is the gram molar mass (GMM)?

SAFETY: Wear safety glasses and aprons.

MATERIALS:

pH meter, 50 ml burette,balance, 100 ml beaker, stirrer, 0.1 M NaOH (aq) and vinegar

PROCEDURE:

1.Determine the mass of an empty 100 ml beaker.

2.Using a graduated 5.0 ml pipette and pipette bulb, add 2.5 ml of vinegar to the beaker and determine the mass of the combined vinegar and beaker system.

3.Add 50 ml of water to the beaker and a few drops of phenolphthalein and a stirring bar.

  1. Fill the empty burette with 0.1M NaOH(aq) as the instructor demonstrates. Make sure you have no air pockets in your burettes from stopcock to the tip. Use waste beaker for this operation as well as Beral pipette or eyedropper to adjust NaOH solution to zero mark on burette.
  2. Set up the entire titration to include the beaker with the vinegar in it; stirring equipment, burette, and pH meter as illustrated by the instructor.
  3. Turn on the stirrer so that there is an even circulation of liquid.
  4. Turn on the previously calibrated pH meter and record the pH of the beaker with the vinegar in it.
  5. Add 1 ml of 0.10 M NaOH(aq) from the burette to the beaker recording the pH after the 1 ml addition each time. If pH meter is fluctuating in readings, turn off the stirrer to take a reading and then turn on stirrer after pH reading is obtained.
  6. When you get to the 17 ml mark of adding NaOH(aq), then add it in 0.5 ml amounts until 21 ml mark is achieved. Then go take 1 ml increments till ending titration at 25 ml.
  7. Using your pH vs. ml of NaOH(aq) added, construct a pH vs. ml of NaOH(aq) added to reach that point. (See below example of typical graph)

pH

ml NaOH added

DATA

ml NaOH addedpHml NaOH added pH

012

113

214

315

416

517

617.5

718

818.5

919

1019.5

1120

20.5

ml NaOH used at equivalence point = _____21

22

23

24

25

CALCULATIONS:

A. Determine the number of moles of sodium hydroxide used.

Since you have sodium hydroxide in solution, you will need to use the molarity (strength of solution and the volume to determine # of moles). Remember there are 1000 ml in a liter.

# moles = (molarity) x (volume in liters)

B. The equation for the reaction is:

CH3COOH(aq) + NaOH(aq) ===> CH3COONa(aq) + H2O(aq) + q

acetic acid sodium hydroxide

From the equation how many moles of acetic acid react with 1 mole of sodium hydroxide?

In your titration, how many moles of acetic acid reacted? (Hint: Look at how many moles of sodium hydroxide were reacted).

C. Determine the mass of acetic acid that was in your vinegar. { you know the moles from part B}. Mass = Moles x GMM

GMM = Gram Molar Mass

D. Determine the mass % acetic acid

Mass % acetic acid = [ mass of acetic acid / mass of vinegar ] x 100

Mass % acetic acid calculated ______mass % acetic acid on label _____

QUESTIONS:

1. How does your % acetic acid compare to the bottle label?

2. Is it necessary to use both a pH meter and an indicator?

  1. What would be the advantage of using an indicator along with the pH meter?
  1. A THOUGHT QUESTION: The results should be the same if one added 35 ml of water to the beaker instead of 30 ml of water. Why is the volume of water not critical?