Errors in pH measurement lead to big errors when calculating % ionisation. That is because the formula
[H+] = 10-pH being an exponential gives big differences in [H+] calculations for small differences in pH
In addition because the pH probes accuracy is +/- 0.05 pH units that alone can lead to large errors in %ionisation calculations. I have calculated the range of % ionisation values for each reading so you can see how a big a difference +/- 0.05 pH error can make. In your summary you need to calculate the [H+] and % ionisation using the pH reading given in the table (but you may wish to note the range).
NB - I went back nd remeasured some of the values after I did my calculations as some were very different - one of the probes was not calibrated correctly and I think that the times that students tested the solutions with the probe not fully submerged made a difference too.
CHEM
|
CONC (mol/L)
|
pH (+/- 0.05)
|
HIGH (%I)
|
LOW (%I)
|
NH3
|
0.1
|
11.6
|
3.5
|
4.5
|
NaOH
|
0.1
|
12.9
|
70.8
|
89.1
|
-
|
-
|
-
|
-
|
-
|
H2SO4
|
0.001
|
2.8
|
141.3
|
177.8
|
HCl
|
0.001
|
3
|
89.1
|
112.2
|
H2SO4
|
0.01
|
1.9
|
112.2
|
141.3
|
HCl
|
0.01
|
2
|
89.1
|
112.2
|
C6H8O7
|
0.1
|
2.1
|
7.1
|
8.9
|
CH3COOH
|
0.1
|
2.7
|
1.8
|
2.2
|
H2SO4
|
0.1
|
0.9
|
112.2
|
141.3
|
HCl
|
0.1
|
1.1
|
70.8
|
89.1
|
HNO3
|
0.1
|
1
|
89.1
|
112.2
|
HCl
|
1
|
0.1
|
70.8
|
89.1
|
Summary
A) Look at your Focus 3 booklet. Identify that pH is a function of [H+]. Identify ALL the factors that can affect pH. Identify specifically how they affect [H+] and thus pH (eg increasing concentration increases [H+] and thus decreases pH). Make sure you don’t make the mistake of saying that increasing the acidity increases the pH. More acidic means a LOWER pH.
B) You need to identify the natural source AND a practical application of each acid and base. Eg CaCO3 being found in limestone is not a ‘use’ of CaCO3
C) You need to explain why all factors that can effect pH must be kept constant if using pH to compare acid strength. This comes straight from our notes.
D) Conclusions are not personal. Conclusions should state the findings – ie what was the order of strength of the four acids?
E) The diagram must have equal numbers of anions and cations. It must have a key and the cations MUST be H+
F) Your table needed to include [H+] with units (which you need to calculate from pH) and the degree of ionisation of each acid (which you also need to calculate).
G) You need to show all equations used for relating [H+] and [OH-] to pH (4 minimum).
H) For the USES of acids in food: identify the acid and what it is used for. Saying acetic acid is in vinegar is not a use – instead you need to say the acetic acid in vinegar is used for…
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