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0BBY1014 Assessed Chemistry Practical 5

13 Mar

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0BBY1014 Assessed Chemistry Practical 5
NAME:
STUDENT ID NO.

71
pH Titrations: Determination of the dissociation constant of a weak
acid
Aims:
1. To study the changes of pH of aqueous solutions of acids when titrated with a
strong base
2. To use measurements on the acids to determine their pK
a value.
Before the laboratory:
Read and understand the following instructions and safety information
Complete the information in the background section below
Look up and record the pKa of propanoic acid (with a reference)
Background:
In aqueous solution, an acid (HA) is a substance that can dissociate to give H3O+ and a
conjugate base, A
.
HA + H
2O H3O+ + A[1]
The strength of an acid is measured in terms of the equilibrium constant, Ka, for the
dissociation represented in equation [1].
K
=
H3O+[A]
[HA] [2]
Where [] indicates the concentration in M.
For a strong acid, Ka is very large; the equilibrium lies towards the right hand side of
equation [1] and the concentration of undissociated acid [HA] is very small.
For a weak acid, Ka is small (much less than 1); the acid is only slightly dissociated and the
concentrations of H
3O+ and Aare much smaller than that of the acid, HA.
The quantity pKa is widely used to describe acid strength. It is defined by pKa = -log
10 Ka
A strong acid has a low pKa value while a weak acid has a high pKa value. Taking logs of
the terms in equation [2] we obtain
log
10 Ka = log10 [H3O+] + log10 {[A] / [HA]}
This can be rewritten as:
pH = pKa + log10 {[A
] / [HA]} [3]
since pH = -log
10 [H3O+].
72
If pH measurements are made during the titration of a weak acid with a strong base, the
ratio [A
] / [HA] can be deduced from the titration data, and this together with the pH values
enables the pKa of the acid to be determined.
When a small amount of a strong base (e.g. NaOH) is added to a solution of a weak acid,
the following reaction occurs:
HA + OH
A- + H2 O
The amount of A
present corresponds to the amount of base added,
When the acid is completely neutralized (this is known as the
equivalence point), the
amount of NaOH added exactly corresponds to the amount of acid initially present. We will
call the amount of NaOH added at this point, Veq .
At an earlier stage in the titration (before the equivalence point has been reached), when a
volume Vt has been added, the ratio [A
] / [HA] must be Vt / Veq – Vt.
When Vt is equal to Veq / 2, half of the acid originally present has been converted to
A
and an equal amount remains as HA.
The ratio [A-] / [HA] is then equal to 1. Since log
10 1 = 0, the pH at this point (the point of half
neutralization), is equal to pKa .
This provides a convenient way of estimating pKa from the titration curve of pH
against volume of base added.
Draw below a sketch of a titration curve for a) a strong acid and strong base and b) a
weak acid and a strong base. Mark on both sketches the endpoint of the titration. On
sketch b mark where you would expect to find the pKa value on the graph.

73
Safety information

Chemical Hazards Precautions
0.1M Sodium
hydroxide
Corrosive Wear gloves, safety glasses and
laboratory coat. If in contact with
eyes, rinse immediately with plenty of
water
0.1 M Hydrochloric acid Corrosive
Irritant
Wear gloves, safety glasses and
laboratory coat. Avoid breathing
fumes. If in contact with eyes, rinse
immediately with plenty of water
0.1 M propanoic acid Corrosive
Irritant
Wear safety glasses, coat and
gloves. If in contact with eyes, rinse
immediately with water. Avoid
breathing fumes.

Experimental procedure:
a) Titration of NaOH with hydrochloric acid (HCl)
1. Place a 100ml beaker on a magnetic stirrer base
2. Place a stirrer bar in the beaker
3. Add approximately 40ml of distilled water (use a measuring cylinder or the
graduation marks on the beaker)
4. Carefully pipette 5ml of approximately 0.1 M HCl into the beaker. DO NOT pipette
from the stock solution; place a suitable amount of acid in a small beaker and take
your sample from this. DO NOT return unused solution to the stock bottle when
you have finished the experiment – pour it down the sink with running water.
5. Turn on the stirrer.
6. Immerse the pH electrode in the acid, making sure it is clear of the stirring bar. Fill
the burette with standard 0.2M NaOH.
7. Clamp the burette above the titration beaker containing the HCl.
8. Before adding any NaOH, measure the pH and record the value in the table below:

Burette reading (ml) pH value
0
0.3
0.6
0.9
1.2
1.5
1.8
1.9
2.0
2.1

74

2.2
2.3

9. Add 0.3 ml increments of NaOH recording the pH in the table above until 1.8 ml of
NaOH has been added.
10. Then add 0.1 ml increments of NaOH, again recording the pH values, until the pH
reaches a value of about 9. The pH will increase sharply at this stage.
11. Revert to 0.3 ml additions of NaOH until the pH has exceeded 10.5. At this point
the titration is complete.
12. Remove the electrode and rinse with distilled water. Check its calibration using the
pH 7 buffer solution
13. Record any drift in the calibration before resetting the meter
14. Remove the stir bar before discarding the contents of the beaker to avoid losing it
b) Titration of NaOH with propanoic acid (C2 H5 COOH)
Repeat the titration following the instructions above but this time use 5ml of approximately
0.1M propanoic acid instead of HCl. Fill your results in the table below and again, check
the calibration of the electrode at the end of the titration.

Burette reading (ml) pH value
0
0.3
0.6
0.9
1.2
1.5
1.8
1.9
2.0
2.1
2.2
2.3

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Practical Report
The practical report for this experiment is a structured lab report which should include the
information and answers to questions found below. You will be assessed on the quality of
your report as well as the of your experimental results.
The deadline for submission is
Wednesday 15th March at 10 pm. Reports must be typed
and submitted via Turnitin to the LBRT 5 assignment.
1. A Title and your student number
2. An Introduction with the following:
a. What are the scientific aims of this experiment? (give your answer in
sentences and don’t use bullet points or lists)
b. Describe the theory that allows you determine an acid’s pKa value from pH
titration measurements. Remember, do not copy word for word from the
experimental protocol or the introduction slides. This counts as plagiarism.
Instead, put things in your own words.
3. An experimental and results section structured as below:
a. In your own words in a written paragraph (no lists), describe how you titrated
NaOH with HCl.
b. Experimental results for part a in table format and graph format (make sure
you plot your graphs as xy scatter plots not line graphs).
c. The calculations (with all working shown) for the concentration of the original
5 mL of HCl.
d. Experimental results for part b in table and graph format
e. The calculations (with all working shown) for the concentration of the original
5 mL of propanoic acid.
f. Calculate the pKa value of propanoic acid using your graph (show how this
was done). Use this result to calculate the
Ka value for propanoic acid.
g. Compare your pKa value for propanoic acid to the literature value obtained
and calculate the percentage error:
% =

4. A conclusion stating any improvements you could make to increase the accuracy
of your pKa value.

 
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Posted by on March 13, 2017 in academic writing, Academic Writing

 

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