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.
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
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Cholesterol levels in serum
Iron in blood
Lead in drinking water
NO in air
Drug tests for athletes
Etc.
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)
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
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
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
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)
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
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Moles of acid = moles of base
Moles of H+ = moles of OHMoles 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
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 5mL NaOH solution into a waste beaker
• Place buret back in buret clamp
Exp 4C: How Much Acetic Acid is in Vinegar?
Experimental: Trial Titration aka Rapid Titration (WHY?)
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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
1) Record the molarity of NaOH
2) Use a funnel to fill the buret, not over the 0.0 line
3) Record the initial volume (SIG FIGS!!!!)
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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)
Exp 4C: How Much Acetic Acid is in Vinegar?
Experimental: Exact Titration
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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
1) Record the molarity of NaOH
2) Use a funnel to fill the buret, not over the top line
3) Record the exact initial volume
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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
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.
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The difference must be not more than 2%.
Calculate the molarity of the vinegar
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