The FeSCN²⁺ formed absorbs light at 447 nm. The amount of light the solution absorbs is directly proportional to the concentration of the solute and the width of the tube holding the sample. This relationship is known as the Beer-Lambert Law. The Beer-Lambert equation is: A = abc, where A is absorbance, a is the molar absorptivity of the solute, b is the wavelength of the light through the sample, and c is the concentration of the solution. Here, the values of a and b are held constant so only A and c need to be considered. Absorbance is measured by a spectrophotometer. If you set the wavelength to 447 nm, you can use the linear graph shown in the book and below to determine the concentration:

You can see the absorbance plotted against concentration FeSCN²⁺ to the 10^-5 power. It is just a matter of reading the graph properly from the absorbance that you get from the spectrophotometer.

 If you look at Run 1 data, you can see that the concentration will be around 2x10^-5 M

This is just an application of the Beer-Lambert law. If you wish to learn more about it, or see the mathematics behind it, visit the web site below and be enlightened.

http://info.xmu.edu.cn/chemistry/chemedu/beerslaw.htm

[Toc][Intro][Procedures][Equations][Beer-Lambert][Credits]