The discussion:
Determining the acid dissociation constant (Ka) for a weak acid.
Results:
Trial run (rough) 1st run (accurate) 2nd run (accurate)
3
Initial reading / cm 0.00 0.00 0.00
Final reading / cm3 25.1 24.4 24.4
Titre (volume used) / cm 25.1 24.4 24..4
3
Mean titre (V) / cm3 24.4
V/2 / cm3 12.2
pH of half-neutralised ethanoic acid solution ( 1st run) 4.72
pH of half-neutralised ethanoic acid solution ( 2nd run) 4.72
pH of half-neutralised ethanoic acid solution ( 3rd run, if required) 4.73
Average pH value 4.72
Calculations :
CH3COOH(aq) CH3COO-(aq) + H+(aq)
[CH3COO-](aq) [ H+](aq)
Ka =
[CH3COOH](aq)
At half neutralising point:
[CH3COOH](aq) = [CH3COO-](aq)
Therefore: Ka = [ H+](aq)
Ka = 10-pH
Ka = 10-4.72
For ethanoic acid: Ka = 1.91x10-5 moldm-3
Comparing my values to published values to determine the accuracy of my measurement of Ka
The published Ka values :
Ethanoic acid = 1.8 x 10-5 moldm-3
My ethanoic acid value is 1.91x10-5 moldm-3 and compared to the published value of the ethanoic acid, it is slightly
different by one decimal place higher. This can be due to the amount of the solution being run slightly longer and run
over the meniscus, which gave inaccuracy. However, using a white pile to determine the desired colour to get also shows
the accuracy for getting a very similar result. as well as calibrating the pH meter correctly and for each use. Hence, the
results show high working standards in the lab, but with only slight inaccuracies that involve the meniscus reading, and
the barrette holding process, which might have been tilting and night straight up. But everything else was accurate, as it
gave extreme similar value results to the published value.
Unit 13: Applications of Inorganic Chemistry/ Assignment 1 / Moreen Mero Page 1 of 19
, Determining the action of an acidic buffer:
Results :
PH of the acidic buffer: 4.31
The volume of Hydrochloric acid / pH reading for HCl pH reading for NaOH
Sodium hydroxide (cm3)
1 drop 4.31 4.37
3 drops 4.31 4.37
5 drops 4.31 4.37
1.00 4.30 4.38
2.00 4.29 4.38
5.00 4.26 4.40
7.00 4.20 4.44
10.00 4.12 4.51
15.00 4.00 4.60
20.00 3.81 5.72
30.00 3.33 4.41
40.00 1.89 5.18
50.00 1.57 5.80
Calculations:
pH = -log Ka + log (conjugate base )
(weak acid)
= -log (1.8x10-5) + log (1.0/2.0)
= 4.74 + (-0.30)
pH = 4.44
The theoretical pH is 4.44
Discussion: comparing the theoretical pH with the experimental pH of the acidic buffer solution:
The pH values of the experiment are quite similar to the theoretical pH value. As the experiment values are lower by one
decimal place. which shows a good level of accuracy during the experiment, which means the pH meter has been
calibrated correctly, and the burette reading was accurate. However, the difference might be the reading of the meniscus
and eye level, and also not being as specific with the reading by not allowing time for the reading to settle, which cause
differences and inaccuracy. Using the right equipment is important, for example, a better measuring scale for each
measuring, that will get validity and more reliable results, as well as not mixing the equipment and using clean ones
because it does affect the accuracy in a very significant way.
Unit 13: Applications of Inorganic Chemistry/ Assignment 1 / Moreen Mero Page 2 of 19
, Buffer action:
A buffer solution, which is known as an aqueous solution, contains either a weak base, and its conjugate acid
(its salt), or a weak acid, and its conjugate base (its salt). Which resist any changes in the pH when adding a
base or an acid. In other words in maintaining and keeping a consistent pH value, when adding a strong base,
the acid that is present in the buffer will neutralize the hydronium ions (OH-) And when adding a strong acid,
the base that is present in the buffer will neutralize the hydronium ions (H3O+). The buffer can be used in so
many ways to regulate and maintain pH, including the buffering system of bicarbonate, which is used for
regulating the pH of the blood in the human body.
An acidic buffer solution as it known it has pH below 7, and are made of weak acid and a salt ( its own salt )
if the weak acid and its salt have the same molar properties, the pH of the solution will be 4. 7, as the
concentration does not matter if they are the same.
An example of acidic buffer solution is mixture of sodium ethanoate and ethanoic acid: as ethanoic acid is a
weak acid, and the equilibrium will be on the left of this position:
By adding sodium ethanoate to it, it will also add more of the ethanoate ions, in which will shift the position
of the equilibrium more to the left. And hence, the solution would contain a lot of ethanoic acid( un-ionised
ethanoic acid), and a lot of ethanoic acid which came from the sodium ethanoate, as well as sufficient hydrogen
ions in the solution to make it acidic.
When adding an acid to this buffer solution, the hydrogen ions will be removed or the pH would be dropping
noticeably. Therefore, the hydrogen ions combine with the ethanoate ions so it will make ethanoic acid. And
as the reaction is reversable most of the hydrogen ions are being removed in this was as ethanoic acid is a weak
acid:
The pH will not change because most of the hydrogen ions are removed, but it will drop and fall a little as this
also involves the equilibrium.
When an alkali is added to this solution, the buffer solution will remove most of the hydroxide ions as the
alkaline solution contain them. And this process is more complex, because there is more than one process to
remove the hydroxide ions.
- First process is removing hydroxide ions by reacting with ethanoic acid: ethanoic acid is very good as
it react and cause more hydroxide ions to be removed as it reacts with acidic substances, and it will
form ethanoate ions and water:
As most of the hydroxide ions will be removed in this reaction, the pH will not increase significantly.
- Second process is removing hydroxide ions by reacting with hydrogen ions: the hydroxide ions will
combine with the hydrogen ions that exist from the ethanoic acid ionisation:
Unit 13: Applications of Inorganic Chemistry/ Assignment 1 / Moreen Mero Page 3 of 19
