According to the <strong>Beer-Lambert Law</strong>,

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<i>A</i> = <i>e</i> &times; <i>b</i> &times;  <i>c</i>

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Where <i>A</i> is the absorbance of a sample, <i>e</i> is the extinction coefficient
of the substance in the sample, <i>b</i> is the path length of the sample (in
cm) and <i>c</i> is the concentration of the compound in solution in the
sample.

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So, in order to find the <strong>dye surface concentration</strong> (not just the
concentration of the dye in the solution), the authors divide the
absorbance of the sample (2.45, unitless) by the extinction coefficient
of the dye.  This yields a number of the form

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<i>bc</i> = <i>length</i> &times;  <i>concentration</i> = (<i>cm</i>) &times;  (<i>mol</i>/<i>cm</i><sup>3</sup>) = <i>mol</i>/<i>cm</i><sup>2</sup> = <i>surface</i> <i>concentration</i>

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Pretty tricky.