For my chemistry class we did a lab on "pKa determination of methyl red indicator", and for my lab report one of the questions asks to calculate the uncertainty (after calculating the average pka, etc.). Normally I would repeat the measurement 6 times to calculate the uncertainty, but due to limited starting materials I only did one complete measurement.
I am really confused on how to go about calculating the propagation of error for uncertainty for my calculations in this lab. (below are my results )
Table 3: Absorbance Values of the indicator solution in buffer solutions at different pH values.
|
Solutions |
Volume of 0.01M CH3COONa (cm3) |
Volume of 0.02M CH3COOH (cm3) |
Indicator Stock (cm3) |
Water (cm3) |
pH |
Absorba-nce at λHMR (at 470 nm) |
Absorbance at λMR (at 421 nm) |
|
1 (red) |
25.0 |
50.0 |
10.0 |
To make up to the mark |
4.67 |
1.961 |
1.230 |
|
2 (blue) |
25.0 |
25.0 |
10.0 |
To make up to the mark |
4.99 |
1.974 |
1.563 |
|
3 (green) |
25.0 |
10.0 |
10.0 |
To make up to the mark |
5.38 |
2.118 |
2.230 |
|
4 (orange) |
25.0 |
5.00 |
10.0 |
To make up to the mark |
5.68 |
1.982 |
2.166 |
AuHMR, λMR = 0.956
AuHMR, λHMR= 2.4654=2.47
AuMR, λHMR =1.8577= 1.86
AuMR, λMR= 2.3181= 2.32
Table 4: Computation of the pKa values of methyl red indicator using Handerson-Hesselbalch equation.
|
S. No. |
pH |
[MR] |
[HMR] |
[MR-]/[HMR] |
log[MR-]/[HMR] |
pKa= pH-log[MR-]/[HMR] |
|
1 |
4.67 |
0.293 |
1.60 |
0.513 |
-0.290 |
4.96 |
|
2 |
4.99 |
0.499 |
1.19 |
1.18 |
0.0703 |
4.92 |
|
3 |
5.38 |
0.882 |
4.53 |
4.53 |
0.656 |
4.72 |
|
4 |
5.68 |
0.875 |
6.04 |
6.04 |
0.781 |
4.90 |
|
Average value of pKa |
4.88 |
|||||
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