1. Exp 4C: How Much Acetic Acid is in Vinegar?
Purpose – To use quantitative analysis and titrations to find the molarity of an acid or base. In this experiment, we shall determine the amount of acid in vinegar.

2. Exp 4C: How Much Acetic Acid is in Vinegar?
Quantitative analysis
Determining the specific amount (molarity or concentration) of a particular substance in a sample
(qualitative analysis determines the presence w/o specifying the amount)
Examples
Cholesterol levels in serum
Iron in blood
Lead in drinking water
NO in air
Drug tests for athletes
Etc.

3. Exp 4C: How Much Acetic Acid is in Vinegar?
Titration
Determining the amount of a substance by a reaction with another substance with known concentration
Detection of the chemical reaction by some kind of signal, e.g., color change of an indicator
Purpose of the Experiment
Determine the amount of acetic acid in vinegar by titration with sodium hydroxide
CH3COOH + NaOH  CH3COONa + H2O
H+ + OH-  H2O
Neutralization Reaction
Indicator is phenolphthalein
Changes from colorless (acidic) to pink (basic)

4. Exp 4C: How Much Acetic Acid is in Vinegar?
Analysis by Titration
Take a known volume of acetic acid
Add sodium hydroxide of a known concentration in small amounts
CH3COOH + NaOH  CH3COONa + H2O
H+ + OH-  H2O
As long as there is more acetic acid than sodium hydroxide, the solution reacts acidic
Indicator will be colorless

5. Exp 4C: How Much Acetic Acid is in Vinegar?
Analysis by Titration (cont’d)
When acetic acid initially in solution equals sodium hydroxide added, you have reached the equivalence point (neutral solution)
When more sodium hydroxide than acetic acid is present, the solution turns basic
Indicator will become pink
When the solution becomes just pink with one drop of NaOH, you have reached the “endpoint” of the titration

6. Point of neutralization
An Acid-Base Titration
Addition of base until all acid is neutralized
Base
Indicator
Acid
Start of titration
Excess of acid
Point of neutralization
Slight excess of base

7. Point of neutralization
An Acid-Base Titration
Point of neutralization
H+(aq) + OH-(aq)]  H2O(l)
Point of neutralization (“Equivalence point”)
All moles of H+ ions present in the original volume of acid have reacted with an equivalent amount of moles of OH- ions form the base added
moles of H+ (originally in flask) = moles of OH- (added from buret)

8. Exp 4C: How Much Acetic Acid is in Vinegar?
Analysis by Titration (cont’d)
Equivalence Point
CH3COOH + NaOH  CH3COONa + H2O
H+ + OH-  H2O
Every single ion of H+ from acetic acid has reacted with an OH- ion from sodium hydroxide to form H2O
Moles of acid = moles of base
Moles of H+ = moles of OH-
Moles of acid = concentrationacid * volumeacid = mol/L * L
Moles of base = concentrationbase * volumebase = mol/L * L
Calculation of acid-base concentrations for a molar ratio of 1:1
Macid * Vacid = Mbase * Vbase
 if you know the volume of acid, the volume of base in a titration and the concentration of one of the components, you can calculate the concentration of the other

9. Exp 4C: How Much Acetic Acid is in Vinegar?
Experimental:Preparation (Different than in lab manual!!! Take notes)
Get 50 mL buret and 10 mL pipet
Measure about 30 mL of vinegar in one beaker and ~ 85 mL of NaOH solution, 0.20 M in another beaker
Place buret in clamp and add about 5 mL of NaOH solution using a funnel
Clean buret by carefully rotating and pouring out the 5-mL NaOH solution into a waste beaker
Place buret back in buret clamp

10. Exp 4C: How Much Acetic Acid is in Vinegar?
Experimental: Trial Titration aka Rapid Titration (WHY?)
Put 5.0 mL of vinegar in a clean 125 mL Erlenmeyer flask
Add ~ 20 mL dH2O
Add 2 drops of phenolphthalein
Fill the buret with 50-mL of NaOH
Record the molarity of NaOH
Use a funnel to fill the buret, not over the 0.0 line
Record the initial volume (SIG FIGS!!!!)
Put the tip of the buret in the Erlenmeyer, and a piece of white paper underneath the flask
Add the NaOH solution in 1-mL increments to the acetic acid while swirling
Note the color after swirling
When you get close to the equivalence point, the solution will briefly turn pink and then colorless again
Add smaller amounts until the solution stays pink for about 30 seconds
RECORD VOLUME OF NaOH ADDED (final reading – initial reading)

11. Exp 4C: How Much Acetic Acid is in Vinegar?
Experimental: Exact Titration
Put 5.0 mL of vinegar in a clean 125 mL Erlenmeyer flask
Add ~ 20 mL dH2O
Add 2 drops of phenolphthalein
Fill the buret with 50-mL of NaOH
Record the molarity of NaOH
Use a funnel to fill the buret, not over the top line
Record the exact initial volume
Put the tip of the buret in the Erlenmeyer, and a piece of white filter paper underneath the flask
Subtract about 1-2 mL from the volume determined in the trial titration
Rapidly add the remainder to the flask
Rinse the wall of the flask with dH2O (Why?)
Continue adding NaOH drop-by-drop until the solution has a barely visible pink color that does not go away upon swirling

12. Exp 4C: How Much Acetic Acid is in Vinegar?
Experimental: Exact Titration (continued)
Record the volume of the buret
The volume added is Vfinal - Vinitial
Repeat the procedure
Calculate the molarity for each procedure using the formula for the titration.
The difference must be not more than 2%.
Calculate the molarity of the vinegar

13. For next week and after:
Due March 26:
1. Results and post-lab for Exp 4C
2. No Pre-lab due
April 2: Long Quiz (See Study Guide given in class, to be posted):
Exp. 1A, 3A, 3B, 4A, 4B
a. Know the purpose and general techniques for each
b. Be able to interpret results
c. Understand and explain safety precautions
d. Study the pre- and post-lab questions
e. Know how to do all calculations