Grade 12 Chemistry
Inquiry: _______ / 33
Communication: ______ / 10
Lab: The Titration of a Weak Acid with a Strong Base
Purpose: To titrate a known volume of vinegar with a known concentration of sodium hydroxide.
To calculate the [HC2H3O2(aq)] of the vinegar and the pH at equivalence
To construct a titration curve and to identify the buffer region.
To measure the pH at specific additions of sodium hydroxide and compare to the calculated pH.
The following chemical reactions will be occurring during the titration:
NaOH(aq) + HC2H3O2(aq)  NaC2H3O2(aq) + H2O(l)
HC2H3O2(aq)
C2H3O2-1(aq) + H2O(l)
C2H3O2-1(aq)
+ H+1(aq)
HC2H3O2(aq) + OH–1(aq)
Materials:125 mL erlenmeyer flasks, burette, burette clamp, funnel, 25 mL graduated cylinder, 150 mL
beaker, white paper, wash bottle, droppers, 0.500 M NaOH(aq), vinegar, phenolphthalein, pH meter
Procedure:
1) Label two erlenmeyer flasks 1 and 2. Rinse the 25 mL graduated cylinder with a small sample of vinegar
and then measure 20 mL of vinegar into each flask. Add one drop of phenolphthalein to each flask.
2) Obtain about 120 mL of 0.500 M NaOH(aq) solution in a 150 mL beaker. Rinse the burette with about
10 mL of this solution and fasten it to the retort stand with the burette clamp. Refill the burette so that the
meniscus of the solution is above the 0 mL mark. Let some of the solution run rapidly from the burette to
expel all air bubbles from the tip. If there is a drop of solution hanging on the tip, remove it.
3) Read and record the initial volume of the NaOH solution in the first observation table. Your eye must be
at the same level as the meniscus.
4) Put the first flask over a piece of white paper, and begin titrating. While continuously swirling the
contents of the flask to ensure thorough mixing, run in the NaOH solution from the buret until a pink colour
persists. This colour change signifies the endpoint of the titration. Near the end of the titration, use your
wash bottle and add partial drops of titrant in order to achieve a slight pink colour instead of a bright fuschia.
Record the final volume of the NaOH solution from the burette in the first observation table.
5) Repeat Steps 3 and 4 with the second flask of vinegar and phenolphthalein. Record your results.
Calculate an average volume of NaOH(aq) needed to reach the endpoint.
6) Use the average volume of NaOH(aq) needed to reach the endpoint in your first titration to decide on
volumes of NaOH(aq) that you will use to construct a titration curve (ie. strategic volumes around the
equivalence point). Be sure to add volumes of NaOH(aq) past the end point as well, to extend your titration
curve past the end point. Record these volumes of NaOH(aq) in the second observation chart.
7) Repeat steps 3 and 4 with a third flask of vinegar and phenolphthalein, but this time, use the pH meter to
record the pH in the flask at the specific volumes of NaOH(aq) that you have chosen. Record the
measured pH in the second observation table.
Observations:
Titration:
Measurements
Volume of Acetic Acid (mL)
Initial Volume of 0.500 M NaOH(aq) (mL)
Final Volume of 0.500 M NaOH(aq) (mL)
Volume of 0.500 M NaOH(aq) Used (mL)
Average Volume of 0.500 M NaOH(aq) Used (mL)
Trial #1
Trial #2
Titration with pH Measurements:
Volume of NaOH(aq) Added (mL)
Measured pH
Calculations:
1) Use your results to calculate the concentration of acetic acid in the vinegar. (4 marks)
2) Calculate the pH of the sample at various additions of the titrant (NaOH(aq)) to fill out the table below.
You should include a sample calculation for the following stages of the titration on an attached paper:
a) 0 mL base added
b) base added before equivalence
c) at equivalence
(5 marks)
(4 marks)
(5 marks)
d ) base added after equivalence
e) table includes at least nine values
(4 marks)
(1 mark)
Titration with pH Calculations:
Volume of NaOH(aq) Added (mL)
Calculated pH
3a) Make a titration curve using the calculated pH vs. the volume of NaOH(aq) added.
Plot the measured pH vs. the volume of NaOH(aq) added, on the same graph.
Create a legend on the graph to differentiate between the two curves. (7 marks)
b) Circle the buffer region on your curve. (1 mark)
Conclusion: (4 marks)
1) State the results of the lab.
2) Discuss two unavoidable errors that could have occurred during the titration (for example: “we measured
incorrectly” is not appropriate as a source of error), that explain any differences on your graph between the
calculated pH and the measured pH.