Determination of pKa and pKb Using Acid-Base Titrations Lab Report
Objectives:
This
experiment aims at determining the concentration of acid-base equilibrium
constants for acetic acid and aqueous ammonia using titration curves.
Introduction:
The
main objective in the volumetric titration of an acid with a base is the
determination of the equivalence point. The equivalence point is a point where
the acid has been fully neutralized by the base. One can determine the
completeness of the reaction by adding an indicator that changes when the
reaction is approaching an equivalence point. Furthermore, the completeness of a
reaction can be determined by a pH meter. A pH meter measures the actual pH of
the solution during titration.
pH titration curves:
In
titration curves of a weak acid or a weak base, the equivalence point lies at
the midpoint of particular two points. Equivalent point is the inflection point
on the titration curve between these points. Once the equivalence point is
established, the volume of a titrant used can be directly read from the curve. Additionally,
pKa and pKb are also read directly from the graph. pKa and pKb are the same as
the pH of the half-equivalence. The titrant volume together with stoichiometry
are used to find the concentration an acid or a base.
Available Materials and Equipment:
- Burets
- Volumetric flask
- Erlenmeyer flasks
- Volumetric pipettes
- Beakers
- 0.100M NaOH
- 0.100M HCl
- Acetic acid
- Aqueous ammonia
- Graduated cylinders
Procedures:
(1)Preparation of Acetic Acid
20ml
of concentrated acetic was obtained and added into a 50ml beaker. A graduated
cylinder was used to add 10ml of distilled water into a 100ml volumetric flask.
5.0ml concentrated acetic acid was pipetted into a 100ml volumetric which
already had 10ml of distilled water. After adding acetic acid, the volumetric was
swirled to thoroughly mix the solution. The volumetric flask was then topped to
the mark with distilled water and covered.
(2) Titration of Acetic Acid with
Sodium Hydroxide
10ml
of distilled water was added into a 250ml Erlenmeyer flask. 20ml of diluted
acetic acid was then added into it, followed by setting up a titration system
with 0.100M of NaOH in buret. A pH meter was used to monitor the pH of the
solution as the base (NaOH) was being added. Sodium hydroxide was added in an
increment of 1ml until the solution pH reached 4.8. After the solution pH
reached, the base was added in an increment of 0.2ml until the equivalence
point was passed. After the equivalence point, the base was added in an internal
of 1ml until there was an insignificant change in the solution pH. The volume
of the base and pH of the solution were recorded and used to draw a titration
curve.
(3) Titration of Ammonia with HCl
Aqueous
ammonia (35ml) was added into a 50ml beaker. 20ml of it was then pipetted into
a 250 Erlenmeyer flask, followed by setting up a titration system with 0.100M
HCl in the buret. As in the case of titration of an acetic acid with sodium
hydroxide, a pH meter was used to monitor the pH of the solution as HCl was
being added. HCl was added in an interval of 1ml until the pH of the solution
was 9. When the solution pH reached 9.5, HCl was added in an increment of 0.2ml
until the equivalence was passed. After this, HCl was added in an interval of 1ml
until there was minimal change in the solution pH. The volume of HCl used and
pH for each addition of HCl was recorded and used to draw the titration curve.
Calculations:
Part 1: Titration of acetic acid with
sodium hydroxide
Graphs:
(1)Using
your titration curve, determine the volume of NaOH required to reach the
equivalence point. Identify this volume and the equivalence point on your
graph.
Answer: 9.3ml.
(2)
Using your titration curve, determine the volume of NaOH required to reach the
half-equivalence point.
Answer: 4.7ml.
(3)
Calculate the concentration (M) of the acetic acid solution.
Answer: 0.0235M.
(4)
Calculate the concentration of original acetic acid.
Answer: 0.094M.
(5)Determine
pKa and Ka of the acetic solution.
pKa=4.3
Ka=5.012 X 10-5
Part 2: Titration of ammonia with HCl
(1)
Using your titration curve, determine the volume of HCl required to reach the
equivalence point. Identify this volume and the equivalence point on your
graph.
Answer: 16.4ml.
(2)
Calculate the concentration (M) of the aqueous ammonia solution.
Answer: 0.0571M.
(3)
Using your titration curve, determine the volume of HCl required to reach the
half-equivalence point.
Answer: 8.2ml.
(4)
Determine the pKa and Ka for the ammonia ion.
Answer:
pKa=4.43, and 3.715 X 10-5.
(5)
Determine pKb and Kb for ammonia.
Answer:
pKb=9.57, and 2.692 X 10-10.
(1)
Write a brief paragraph your two titration curves. You should discuss the
similarities and the differences between the components of the solutions at
each of the following points. You should write equations and use your data to
support your answers.
(a)
Between the initial pH and equivalence point
(b)
At the equivalence point
(2)
Compare the similarities or differences of your curves along x-axis and y-axis.
(3)
The accepted values for the pKa and Ka of acetic acid are 4.74 and 1.8 X 10-5
respectively. How do your experimentally determined values compare to accepted
values? Percent error is a good way to make this comparison.
(4)
The accepted values for pKb and Kb are of ammonia are 4.75 and 1.76 X 10-5
respectively. How do your experimentally determined values compare to accepted
values? Percent error is a good way to make this comparison.
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