Titration of Acetic Acid
Prepared by Thomas M. Moffett Jr., Plattsburgh State University, 2003
Objective: Determine the concentration of acetic acid by titration with a standard sodium
hydroxide solution.
Background: Review acid/base titrations in your textbook.
A weak acid is a compound that partially ionizes in aqueous solution producing
hydronium (H3O+) ions. The general equation for the dissociation of any weak acid can
be written as:
HA (aq) + H2O (l) ⇔ A- (aq) + H3O+ (aq)
(1)
The addition of a strong base results in a neutralization reaction in which hydroxide ions
(OH-) react with hydronium to produce water:
H3O+ (aq) + OH- (aq) → 2 H2O (l)
(2)
As hydronium is consumed in the neutralization reaction, the equilibrium in equation 1 is
shifted to the right according to Le Chatelier’s Principle. The overall reaction for the
neutralization process can be written as the sum of equations (1) and (2):
HA (aq) + OH- (aq) → A- (aq) + H2O (l)
(3)
The concentration of the unknown solution can be determined by measuring the volume
of titrant added to reach the equivalence point. The equivalence point occurs when all of
the acid has been neutralized by the base. At the equivalence point:
MAVA = MBVB
(4)
Where MA is the molarity of the acid, MB is the molarity of the base and VA and VB are
the volumes of the acid and base respectively. The equivalence point for the titration will
be determined by using an indicator that changes color at the equivalence point.
Phenolphthalein is pink in bases and clear in acids. When the titration mixture changes
from clear to pink, the equivalence point has been reached.
Procedure: Safety goggles must be worn at all times in the laboratory.
1.)
Obtain about 35 mL of the acetic acid solution in a clean dry Erlenmeyer
flask. Also obtain about 70 mL of the sodium hydroxide solution in another
clean dry Erlenmeyer flask (be sure to record the concentration).
2.)
Rinse a 50 mL buret with two separate 5 mL portions of sodium hydroxide.
3.)
Fill the buret with the NaOH, noting the starting volume.
4.)
Rinse a 25 mL pipette with a small portion of the acid solution.
5.)
Pipette a 25.00 mL acid sample into a clean dry 150 mL beaker. Add three
drops of phenolphthalein to the beaker.
6.)
Slowly add one to two mL of NaOH to the acid, stir. Continue adding NaOH
in one to two mL increments. When the pink color flashes through the entire
sample, add NaOH drop wise.
7.)
When the solution remains pink you have reached the equivalence point,
record the volume of NaOH.
8.)
Record the pH of your solution at the equivalence point.
9.)
Repeat with two additional trials.
Name: _________________________________
Titration of Acetic Acid – Data Sheet
Section: ____
CHE 101
Molarity of NaOH solution:
_________
Volume of Acid
_________
_________
_________
Final Buret level
_________
_________
_________
Initial Buret level
_________
_________
_________
Volume of NaOH
_________
_________
_________
pH at Equivalence
point
_________
_________
_________
Molarity of Acid
_________
_________
_________
Average Molarity
_________
%-Error
_________
Calculations (show work below)
Name: _________________________________
Titration of Acetic Acid - Questions
Section: ____
CHE 101
1.) Write the balanced net ionic reaction that occurs between acetic acid and sodium
hydroxide.
2.) The pOH of a solution is 3.78, determine pH, [H3O+], and [OH-]. Is the solution
acidic, basic, or neutral.
3.) Explain why the pH at the equivalence point is not 7.00.
4.) Write a balanced net ionic reaction for the titration of nitric acid with potassium
hydroxide.
Name: _________________________________
Titration of Acetic Acid – Pre Lab Questions
Section: ____
CHE 101
1.) What are the Arrhenius definitions of an acid and a base?
2.) What are the Brønsted-Lowry definitions of an acid and a base?
3.) Write a reaction showing the autoionization of water.
4.) Hydrofluoric acid and sodium hydroxide react according to the following reaction:
HF (aq) + OH- (aq) → F- (aq) + H2O (l)
How many milliliters of 0.250 M sodium hydroxide are needed to titrate 35.0 mL of
0.425 M hydrofluoric acid?