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Online Lab Experiment AB-251 Acid-Base Titration Using a pH Meter
For background and explanations, refer to the Introduction to this experiment on page 73 of the
Chem 142 lab manual. Actual details of the experiment are included below, including a useful
prelab demonstration video. Also refer to Ch 17 section 3 (17-3) of the 9th Edition of your Kotz
textbook and your lecture notes for Ch 17.
Procedure: Two titrations were carried out. In each titration, a precisely known volume of acid
was placed in a beaker and NaOH of precisely known concentration was added to it from a buret.
A pH meter placed in the acid solution was used to record pH of the solution as each addition of
NaOH was made.
A. Titration of a strong acid with a strong base (NaOH).
25.00 mL of hydrochloric acid of unknown concentration was placed in a 100 mL beaker and
titrated with 0.120 M NaOH from a buret. The NaOH was added in small increments (approx. 0.5
mL each time) and the pH of the solution, and total volume of NaOH added, was recorded after
each addition of NaOH.
B. Titration of a weak acid with a strong base (NaOH).
25.00 mL of acetic acid of unknown concentration was placed in a 100 mL beaker and titrated
with 0.120 M NaOH from a buret. The NaOH was added in small increments (approx. 0.5 mL
each time) and the pH of the solution, and total volume of NaOH added, was recorded after each
addition of NaOH.
Prelab demo video: The following video (approx. 17.5 minutes, available on youtube) contains a
demonstration of an experiment very similar to that you would have performed in lab (although it
uses KOH instead of NaOH). It also has a pretty good explanation of part of the data analysis (how
to determine the equivalence point).
Data and Observations: Available in a different assignment in the Ch 17 module in Canvas.
For each titration, the data consisted of pH values and total volume of NaOH added, in mL. This
data was plotted as pH (y-axis) vs volume of NaOH added (mL, x-axis).
Download a separate PDF file from an assignment called “AB251 Data file xx†(where xx is a
number from 01 to 08). You will find this assignment by clicking on the “Assignments†link in the
Canvas menu (in the “Upcoming Assignments†section, you should see only one such AB251 Data
file assignment). It will direct you to download the PDF file. The downloaded PDF will contain
plots showing the pH vs the volume of NaOH added (mL) for each of the titrations. The plots are
identified by a title (Strong acid-strong base or Weak acid-strong base).
Post Lab Questions (these are not the same as in the Chem 142 lab manual).
1. For the strong acid titration do the following:
a) Print the graph of pH vs volume NaOH added for the strong acid-strong base titration.
b) From the graph, determine the pH and the volume of NaOH added at the equivalence point.
Show on the graph itself how you determined these points (draw horizontal and vertical lines from
the equivalence point in pencil) and indicate on the graph what values you obtained. Refer to figure
17.4 on page 644 of the textbook (9th Ed.), and the prelab video (above) for help with this. Scan or
take a picture of this annotated graph to include in your lab report.
c) Calculate the molarity of the hydrochloric acid using the volume of NaOH added at the
equivalence point. Show the steps in your calculation.
2. For the weak acid titration do the following:
a) Print the graph of pH vs volume NaOH added for the weak acid-strong base titration.
b) From the graph, determine the pH and the volume of NaOH added at the equivalence point.
Show on the graph itself how you determined these points (draw horizontal and vertical lines from
the equivalence point in pencil) and indicate on the graph what values you obtained. Refer to figure
17.5 on page 647 of the textbook (9th Ed.), and the prelab video (above) for help with this.
c) Calculate the molarity of the acetic acid using the volume of NaOH added at the equivalence
point. Show the steps in your calculation.
d) From the graph, determine the pH and the volume of NaOH added at the half-equivalence
point. Use the volume of NaOH added at the equivalence point (Q2b) to calculate the halfway
point volume (volume of NaOH added to get halfway to the equivalence point = volume NaOH
added at equivalence point / 2 ), then determine the pH at this halfway point from the graph. Show
on the graph itself how you determined these points (draw horizontal and vertical lines from the
half-equivalence point in pencil) and indicate on the graph what values you obtained. Refer to
figure 17.5 on page 647 of the textbook (9th Ed.) for help with this. Scan or take a picture of this
annotated graph to include in your lab report.
e) Calculate the Ka of acetic acid. Read the information about titration of a weak acid with a strong
based on page 647 of the textbook (9th Ed.) for help with this.
f) Compare your experimental value for the Ka of acetic acid to the true value for acetic acid by
calculating a percent error. The true Ka for acetic acid is 1.8 × 10-5.
3. Assume that the strong acid used in the titration in part A was sulfuric acid instead of
hydrochloric acid. Based on the data you analyzed, calculate the concentration of the sulfuric acid.
4. For the titration of the acetic acid with NaOH, calculate the expected pH of the solution at the
equivalence point.
Hint: You will need to use the volume of acetic acid (from procedures information), the volume
of NaOH at the equivalence point (Q2b), and the calculated concentration of acetic acid (Q2c) in
your answer. You will also need the Kb value for acetate ion (CH3COO-). Start by calculating the
concentration of acetate ion at the equivalence point. You will then set up an ICE table based on
the equilibrium equation for the acetate ion acting as a weak base. From this you will be able to
determine the concentration of OH- ions and therefore pOH and then pH. Refer to example
problem 17.6 (part b) on pages 648 – 650 of the 9th Ed of the Kotz textbook for guidance on the
procedures involved in this calculation.
Lab Report
Title Page
Scans/Pictures of annotated graphs (make sure details can be seen)
Answers to postlab questions 1 – 4 above. You may type or hand write your answers. Rewrite the
questions before you answer them. Show steps in calculations for full credit.
Try to combine these materials into one file if possible. Please name all files following the naming
convention “Lastname_Initial_AB251â€Â.
Strong Acid – Strong Base
S+
P/
Weak Acid – Strong Base
S+
P/
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