Despite this problem misses out crucial information for its solution, it turned out possible to come up with the attached absorbance graph, which shows the behavior of the absorbance of the sample with respect to the concentration at 453 nm.
As you can see, there is a red line and circle pointing out the concentration at which the sample has an absorbance of 0.20 (y-axis) which turns out to be 4x10⁻⁵ M (x-axis).
Now, since we require the moles of the iron (III) ions present in 100. mL of the sample, we recall the concept of molarity, which requires moles and volume in liters:
[tex]M=\frac{n}{V}[/tex]
Thus, since all the Fe⁺² in FeSCN⁺² is converted to Fe⁺³ we can find out the moles as shown below:
[tex]n=4x10^{-5}\frac{mol}{L} *0.100L\\\\n=4x10^{-6}mol[/tex]
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