The Richard Stockton College of New Jersey CHEM 2115: General ...

4. Since we know the initial concentration of each reactant (the I-row) and we know the change in
concentration to get to equilibrium (C-row) we can find the equilibrium concentration of each reactant
in the following manner:
[F e +3 ]eq = [F e +3 ]initial − x = [F e +3 ]initial − [F eSCN +2 ]eq
[SCN − ]eq = [SCN − ]initial − x = [SCN − ]initial − [F eSCN +2 ]eq
We can repeat this calculation to determine the equilibrium concentration of each reactant for each
sample (1–5).
5. We now know all three of the equilibrium concentrations so we can calculated Keq for each sample:
Note on Standard Solutions
Keq =
[F eSCN +2 ]eq
[F e +3 ]eq[SCN −1 ]eq
In preparing the standard solution you were assigned a volume of 0.00100 M KSCN to place in your 50 mL
volumetric flask. You then added 15 mL of 0.200 M Fe(NO3)3 and diluted to a total volume of 50mL.
Since you are adding a very large excess of iron (III) nitrate, the limiting reactant is KSCN and you can
assume that since there is a huge excess of iron ions, that the reaction goes to completion. Therefore, the
concentration of FeSCN +2 in your standard solution is equal to the initial concentration of KSCN.
Again this leads to a dilution calculation:
[F eSCN +2 ]standard =
where VKSCN is your assigned volume of KSCN in LITERS.
(0.00100 M)(VKSCN)
0.050 L
After entering the data in Excel, you will need to prepare the standard curve, which is a plot of the Absorbance
(Abs) on the y-axis and [FeSCN +2 ]standard on the x-axis.
You will need to fit a line to the data with the equation displayed on the graph. It is this equation that you
will use to convert the absorbance measurements on samples 1–5 to [FeSCN +2 ]eq (see step 2 above).
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