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Unit 13 A1 Learning aim A: Investigate acid-base equilibria in order to understand buffer action and to optimise acid-base titration procedures
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Results table of Ka for a weak acid & CalculationsDetermining the acid dissociation constant (Ka) for a weak acid.
Results:
Trial run (rough) 1st run (accurate) 2nd run (accurate)
Initial reading / cm
3
0.00 0.00 0.00
Final reading / cm3 25.1 24.4 24.4
Titre (volume used) / 25.1 24.4 24..4
cm 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
Task 2: Comparison of the actual vs theatrical values
pH calibrated and the equipment and what things impacted & the titration errors. Not using a white tile and if
the colours are dark and if added more than required and if it has air bubbles and the pH meter if not
calibrated well but not patting it dry with a paper towel to remove excess moisture and the funnel at the top
of the burette the liquid could have fallen in more which could have altered the readings of the Ka value. Not
mixing the mixture well enough so that its evenly mixed. Also, going over the meniscus line which this
could have been an error for the readings and added more hydrochloric acid that’s in the beaker and the pH
meter bulb wasn’t as inside measuring the hydrochloric acid. The pipette filler with the pipettes it went over
the 25cm3. The amount needed for phenolphthalein with the drops it went over the needed drops of 3x. so,
this caused an inaccuracy of the experiment. The end point error can have these types of issues when it
comes to interpreting because the colours can change fast depending on if the amount was enough to create a
colour change. Human errors and sometimes the equipment might have errors which can cause from the
burette since if the tap isn’t closed little drops can cause changing of the readings. Published Ka value for
ethanoic acid is 1.8 x 10-5 moldm-3 and for ethanoic acid: Ka = 1.91x10-5 moldm-3.
Figure 1: Experiment of titration using indicator of phenolphthalein
Unit 13: Applications of Inorganic Chemistry Page 1 of 8
, Task 3: Compare the theoretical pH with experimental pH of the acidic buffer solution
Results
pH of the acidic buffer 4.37
VOLUME OF pH Readings for HCL pH Readings for NaOH
HYDROCHLORIC
ACID/SODIUM HYDROXIDE
(cm3)
1 drop 4.31 4.37
3 drops 4.31 4.37
5 drops 4.31 4.37
1.00 ml 4.30 4.38
2.00 ml 4.29 4.38
5.00 ml 4.26 4.40
7.00 ml 4.20 4.44
10.00ml 4.12 4.51
15.00ml 4.00 4.60
20.00ml 3.81 4.72
30.00ml 3.33 4.91
40.00ml 1.89 5.18
50.00ml 1.57 5.80
Calculation
pH = -log Ka + log (conjugate base)/(weak acid)
pH = -log(1.8x10-5) + log 1.0/2.0
= 4.74 + (-0.30)
= 4.44
Theoretical pH is 4.44
The buffer solution was close, and conclusions made from the results are that explains a buffer action of your
solution and assess the limits of this action and how does your acidic buffer act when adding H+ and a base
Unit 13: Applications of Inorganic Chemistry Page 2 of 8
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