For my chemistry class we did a lab on \"pKa determination of methyl red indicat
ID: 907016 • Letter: F
Question
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 HMR (at 470 nm)
Absorbance at MR (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, MR = 0.956
AuHMR, HMR= 2.4654=2.47
AuMR, HMR =1.8577= 1.86
AuMR, MR= 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
Solutions
Volume of 0.01M CH3COONa (cm3)
Volume of 0.02M CH3COOH
(cm3)
Indicator Stock (cm3)
Water (cm3)
pH
Absorba-nce at HMR (at 470 nm)
Absorbance at MR (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
Explanation / Answer
SOLUTION:
Graphs should be at least one-third page in size with axes labeled and with units. Expand
the axes, if necessary, so that your curve fills the plot (i.e. no large rectangles of empty space in
the plot). Slopes and intercepts from curve fitting should always be given with uncertainties.
Include the uncertainty for the the pKa' values propagated from the curve fit values (see the Error
Analysis handout for instructions for representing uncertainties). (You do not need to propagate
the uncertainties of the volume measurements through to the final results. Just start with the
uncertainties in the fit coefficients).
Estimating the Expected Error in the Final Result Based on the Measurement Errors:
You can estimate an upper bound for the expected error in the result by using just one data point.
Least squares curve fitting will give a smaller error, since the result is based on multiple trials,
but doing the calculation with just one data point will give an upper bound for the final error. The
uncertainty in absorbance measurements is ±0.002 at best. Since using the Beer-Lambert Law for
calculating concentrations involves multiplication and division, the errors in the concentrations
from the absorbances propagate as relative errors (even though we used some matrix tricks in the
process). In other words, assume the relative errors in the concentrations result from the relative
errors in the number of absorbance measurements required to calculate the concentrations. The
uncertainty in pH measurements is ±0.03, unless extra care is taken.
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.