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Experiment
8
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Spectrophotometric Determination of Formula
Overview
In this experiment, you prepared nine
different mixtures of solutions of iron(III) and 5-sulfosalicylic
acid (SSA), and measured the % Transmittance of these mixtures with
the Spectronic 20 spectrophotometer (the product of the reaction is
colored purple). The mixtures were prepared systematically, from the
first mixture (in which there was a small amount of iron and a large
amount of SSA present) to the last mixture (in which there was a
large amount of iron but only a small amount of SSA present). You
calculated the absorbance of each solution (from its %
Transmittance) and then made a graph in which you plotted these
absorbances versus the mole fraction SSA for each solution.
The graph should show a maximum, which represents the molar ratio in
which iron(III) and SSA react.
When two chemical substances react with
each other, they react in a certain fixed, definite mole ratio (the
stoichiometry of the reaction. When you prepare the set of
systematically-varied mixtures of SSA and iron(III), only one
of the mixtures reflects the correct mole ratio for the
substances to react to the maximum extent possible. The mixture in
which the substances have reacted to the maximum extent possible has
the deepest purple color (largest amount of product formed).
Calculations
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Concentration of
Stock Solutions |
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Fe(III)
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SSA
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1.65 × 10-3
M |
1.71 × 10-3
M |
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Test Tube #
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%T
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1
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61.0% |
2
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39.4% |
3
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27.7% |
4
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22.1% |
5
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19.9% |
6
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22.0% |
7
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27.9% |
8
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40.1% |
9
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60.5% |
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Page 47, Part II
A. Calculation of Absobance of
Solution in Test tube #3
It is the absorbance of a colored
solution which is directly related to the concentration of the
colored species in the solution. The absorbance is calculated from
the measured % Transmittance (a derivation follows):
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Absorbance = -log
(%T/100) = log (100/%T) = log (100) - log (%T) = 2 - log(%T)
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Be very careful how you use your
calculator for this calculation. Some calculators require different
keystrokes for using the logarithm of a number in a calculation.
Absorbance values should come out to be between 0 and 2: if you get
any other sort of number, you are using your calculator
incorrectly!!
For solution #3, the % Transmittance
was 27.7%. The absorbance corresponding to this is
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Absorbance = 2-
log(%T) = 2 - log(27.7%) = 2 - 1.442 = 0.558 |
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B. Calculation of the moles of Fe+3
is Test Tube #3
To calculate the number of moles of
iron(III) present in Test tube #3, you need two bits of information.
You need to know the volume of the stock Fe(III) taken as
well as the concentration of the Fe(III) stock solution.
On Page 43 at the bottom is the volume
of iron(III) that was to be placed into each of the test tubes. For
Test Tube #3, you should have used 3.00 mL of the iron(III) stock
solution.
You should have recorded the exact
concentration of the iron(III) solution from the bottle label on the
data page (Page 47, top). For my data above, the concentration of
iron(III) is 1.65 × 10-3 M (M means moles
of Fe3+ per liter).
Given this, calculate the number of
moles of Fe(III) in Test Tube #3, then click here
to check your answer.
C. Calculation of the moles of SSA in
Test Tube #3
This calculation is similar to that in
Part B, only for SSA in Test Tube #3 (rather than Fe3+).
On Page 44 is a table which indicates the amount of SSA solution you
were supposed to have used for each mixture: for Test Tube #3, you
should have used 7.00 mL.
You should have recorded the
concentration of the stock SSA solution in your data: for my table
of data above, the concentration of SSA is 1.71
× 10-3
M
Given this, calculate the number of
moles of SSA in Test Tube #3, then click here
to check your answer.
D. Calculation of mole fraction of SSA
in Test Tube #3
The mole fraction of a given component
in a mixture represents the number of moles present of the
component of interest, divided by the total number of moles
of all components present. We have a mixture containing 4.95
× 10-6
moles of Fe3+ and 1.20
× 10-5 moles of SSA.
Using this data. Calculate the mole
fraction of SSA in Test Tube #3, then click here
to check your answer.
E. Calculation of mole fraction Fe(III)
in Test Tube #3
This calculation is based on the same
data as in Part D above, only in terms of Fe(III) rather than SSA:
Alternatively, since there are only two
components in the mixture, and since the sum of all the mole
fractions of the components in a mixture must equal 1,
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XFe = (1
- XSSA) = (1- 0.708) = 0.292. |
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Page 48, Part III
A. Summary of Results
This table just summarizes all your
results for Absorbance and Mole Fraction for each of your solutions.
The calculations are done exactly as illustrated above, only
substituting the correct volumes of Fe and SSA for the individual
solutions.
B. Results from Graph
When plotting your graph, be sure to
follow the instructions for graphing in the Appendix to the lab
manual. When you plot your nine data points, you should notice that
there is a group of ascending data points and a group of
descending data points. In addition to drawing a smooth curve
through the data points, you should plot two lines on your graph:
one which encompasses the ascending data points and one which
encompasses the descending data points. The intersection of these
two lines more easily shows the location of the maximum point on
your graph (relative to the mole fraction, horizontal scale).
Here is a crude example of the
general shape of the graph (your own graph should reflect
your own data and should be drawn more carefully).
1. This point is read off the graph.
2. This can be read off the graph, or
you can realize that XFe = (1 - XSSA)
3. This represents the answer to 1
divided by the answer to 2
4. This represents the ratio determined
in 3 rounded to the nearest whole numbers.
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