Determining the Concentration of an Unknown Solution CoCl2 6H20 Using Spectroscopy
Data Collection:
First, we must determine the λmax. We must use the λmax for the experiment. We found the
λmax by adjusting the wavelength to find the wavelength that has the highest absorption with the
lowest transmission.
Absorption Spectrum
Absorption
0.07
0.42
0.81
0.36
0.11
0.07
Wavelength (nm)
400
450
500
550
600
650
0.9
Transmission
79
35
16
43
83
83
Absorption Rate at Different Wavelengths
0.8
0.7
Absorption
0.6
0.5
0.4
0.3
0.2
0.1
0
400
450
500
550
600
650
Wavelength (nm)
As shown in the graph above, the absorption peaks somewhere between 500 – 525 nm. We can
now zero in and use the absorption spectrum at every 10 nm to find a more precise λmax.
Absorption Spectrum at 10nm
Wavelength (nm)
Absorption
Transmission
490
0.78
17
510
0.85
15
520
0.79
16
From the chart above, we can see that the wavelength 510nm gives the highest absorption and
the lowest transmission. This means that we will be using this transmission for the rest of the
experiment.
Our test tube #1 was 100% of our cobalt chloride solution. Test tube #2 had 80% of our cobalt
chloride solution. Test tube #3 had 60% of our cobalt solution and so on. Our cobalt chloride
solution was made to be a 0.14 molarity solution.
Tube #
1
2
3
4
5
6
Using Beer’s Law
Concentration (molarity)
0.14
0.112
0.084
0.056
0.028
0
Absorbance at 510nm
0.89
0.72
0.55
0.40
0.19
0.00
We made our stock solution by figuring out the molar mass of CoCl2 H2O
The molar mass of CoCl2 H2O = 237.93 g/mol
We want a 0.14 molarity and since we are using 100mL, the molarity becomes 0.014
0.014 x 237.93 = 3.33 g
Therefore we need 3.33grams of cobalt chloride but we made 3.34 grams because it was hard
to get that precise.
(3.34-3.33)/3.33 = 0.003 or 0.3% error.
Absorpance of Light Compared to the Concentration of the
Cobalt Chloride Solution (Caibration Curve)
1
y = 6.474x
0.9
0.8
Absorpance
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
Concentration (molarity for 100mL)
A= *b*c
Using this formula (Beer’s Law), we should see a linear relationship between the
absorbance and the concentration. This is because b is negligible because we are using curvettes and
they are typically 1 cm in width, so it is often left out of the equation. This leaves A = * c
absorbance = some constant multiplied by the concentration. Using excel, I graphed the absorbance
versus the concentration and with a line of best fit, I found the constant to be 6.474
I can now find unknown concentrations and unknown absorptions given the absorbance or
concentration respectively. I was given the unknown solution B which had a absorbance of 0.7 Looking
at the graph, a concentration of 0.115 molarity in a 100mL solution would have this absorbance.
Conclusion:
As seen in the graph above (Concentration vs. Absorbance), there is a linear relationship
between the concentration of the solution and the absorbance of the solution. This supports
what we know. We know that there should be a linear relationship between concentration and
absorbance and this graphs shows that. Using an online resource, I found the molar absorptivity
constant of CoCl2 6H2O to be approximately 5
. 1Using excel, I found the slope
of my graph that compares concentration and absorbance to be approximately 6.474 I can use
these values to calculate a percentage error.
1
http://answers.yahoo.com/question/index?qid=20130111175717AAZjjHr (link on this website)
0.16
(6.474 – 5.000)/ (5.000) = 0.295 = 29.5%
Therefore, the molar absorptivity constant that I calculated from this experiment is
approximately 29.5% greater than the literature value. This percentage error is acceptable
although it tells us that this lab has many limitations and that many improvements could be
made. We could have calculated a percentage error using the concentration that we calculated
from the graph with the actual concentration of that unknown solution, but we were not given
the actual concentration, so we could not calculate it. This is why I substituted the calculation of
molar absorptivity constant as my percentage error, because these are values that we have.
Overall, this experiment was decently well done, with a percentage error of 29.5 but there are
limitations and improvements that could be made to this lab.
Evaluation:
There are limitations to this experiment such as the lack of trials. There was a serious lack of trials as we
only performed this experiment once. Once is never enough for an experiment. Our results could have
been skewered or a fluke this time. We should definitely try to do more times so that we could find an
average or figure out if the data is consistent. 5-10 trials is a suitable number of trials to do.
Another limitation to this lab is that in between trials, the same equipment was used and this could have
caused cross contamination or a variation in concentration. For example, after the 100% solution, we
used the same test tube for the 80% solution. Since the test tube was not completely empty and a few
droplets were still left, the concentration or molarity of the solution for the next trial could have
changed slightly. This same problem occurred with the curvettes. When we were putting our different
solutions in the curvette, only one curvette was used and it was not washed before the next solution.
An improvement would be to wash out all test tubes and curvettes after each usage or have a bunch of
clean new ones ready to be used.
As calculated above, there were measurement errors such as the weight of the cobalt chloride. Also,
there were errors on our equipment such as 100 mL ±16 for the test tubes. An improvement to this lab
would be to use more accurate equipment. For example, micropipettes could have been used to
measure out 100mL of solution exactly. And used out how much water we need to dilute the cobalt
chloride. This would have made our solutions and calculations much more accurate.
Another limitation to this lab would be that we tried to find the λmax to the nearest 10s. We could have
been more specific and tried to find it to the nearest 5. Looking at the data, we can see that even raising
or lowering the spectrum would have a drastic effect on the absorbance which is why 510 nm might not
be the λmax but rather 515 nm or 505nm might be. For next time, we could have tried to find a more
precise λmax which would have definitely affected the calibration curve which in turn affected what
concentration we get for a 0.70 absorption.