CHM-102
Titrations: Determination of the Molarities
of Strong and Weak Acids
General Chemistry 101/102
Laboratory Manual
University of North Carolina at Wilmington
UNCW
CHM-102
Titrations: Molarities of Strong and Weak Acids
•
Purpose
 To study the differences between strong and weak acids by
titration with a strong base.
•
Safety Considerations
 Safety glasses must be worn throughout the entire
experiment, even when cleaning glassware to prevent
damage due to strong acids and bases.
 Be cautious when using unknown solutions and clean up
any spills immediately. Contact with strong acids or bases
will affect skin and create holes in clothing. Wash the
affected area thoroughly if you suspect any skin contact any
of unknown solutions.
 Flush all wastes down the drain with plenty of water.
UNCW
CHM-102
Titrations: Molarities of Strong and Weak Acids
•
Procedure
• Procedure
 Connect the Tablet
computer, Microlab
unit, and the pH
probe as shown in
the photo.
UNCW
CHM-102
Titrations: Molarities of Strong and Weak Acids
•
Procedure
 To begin determining pH and volume measurements on the
computer, click on the Applications folder, and select the
Microlab prompt. Then double click the “pH Titrations”
prompt as shown below.
UNCW
CHM-102
Titrations: Molarities of Strong and Weak Acids
•
Procedure
 Click on the experiment entitled “pH Acid-Base –
Temperature Manual Titration”, as shown below.
UNCW
CHM-102
Titrations: Molarities of Strong and Weak Acids
•
Procedure
 Current pH values
will be displayed in
the lower right hand
corner of the screen.
 Values of pH
corresponding to
volume of titrant
added will be
collected and
displayed in the table
on the computer
screen as shown in
the photo.
UNCW
CHM-102
Titrations: Molarities of Strong and Weak Acids
•
Procedure
 To begin the
titration, click on
the “Start”
prompt.
UNCW
CHM-102
Titrations: Molarities of Strong and Weak Acids
•
Procedure
 Before you begin your
titration, enter the initial
volume as 0 mL.
 Press “Enter and
Continue” to allow the
computer to record the
corresponding initial pH
value of the unknown
acid.
UNCW
CHM-102
Titrations: Molarities of Strong and Weak Acids
•
Procedure
 Add 1 mL of base
(titrant) to your
unknown acid solution.
 For best results, place
the tip of the burette
inside and along the
side of the beaker,
allowing the titrant to
run down the side of
the beaker and into the
solution (rather than
hitting the pH electrode
or splattering).
UNCW
CHM-102
Titrations: Molarities of Strong and Weak Acids
 After the 1 mL of base (titrant) has
been added, swirl the solution with
the electrode. In the keyboard
input enter the value of “1” for
volume in mL and press “Enter and
Continue” to record the pH value.
 Continue to add 1 mL of titrant at a
time. Stop after each addition of
titrant to stir the solution and
record the volume value and the
corresponding pH value.
 Enter the volume values in
sequence (1 mL, 2 mL, 3 mL, 4
mL, 5 mL, etc.) in the input box on
the computer even though only 1
mL of titrant is added at a time.
UNCW
CHM-102
Titrations: Molarities of Strong and Weak Acids
 Add 1 more mL of titrant when
the solution turns dark pink and
remains dark pink after swirling.
Click the “Stop” prompt. Remove
the pH probe immediately and
rinse with DI water. Place the
electrode in pH=7 buffer solution
until next use.
UNCW
CHM-102
Titrations: Graph of Strong Acid
 The graph of a strong acid will display a sharp curve indicating equivalence.
The midway point of the curve is the equivalence point, where the
stochiometrically equivalent number of moles of base (titrant) equals the
number of moles of acid in the unknown solution. From this point the
equivalence volume and equivalence pH can be determined on the respective x
and y axes.
Equivalence point
Equivalence pH
Equivalence volume
UNCW
CHM-102
Titrations: Graph of Weak Acid
The graph of a weak acid will display a gentler curve where the
midway point indicates equivalence point. The pKa = pH at half the
equivalence volume as shown below in black lines. (13 mL/2 = 6.5 mL)
Equivalence point
pH = pKa = 4.75
½ the Equivalence volume = 6.5 mL
UNCW
Equivalence volume = 13 mL
CHM-102
Titrations: Molarities of Strong and Weak Acids
•
Calculations
 To determine the concentration of the unknown acid (Molarity),
we can use the following formula:
Moles of unknown acid in the flask = moles of base (titrant added)
Molarityacid x volumeacid = molaritybase x volumebase

Molarityacid =
molaritybase x volumebase
volumeacid
Where volumebase = equivalence volume determined from the graph
(NOTE: Volumebase is not the total volume of base added, nor the volume
of base added when the solution turned pink).
UNCW