Question

The extinction coefficient for FD&C Blue Dye #1 is 138,500 cm-L/mol at 630 nm. A chemist collected the following data at 630 nm. She also found that the absorbance of an FD&C Blue Dye #1 solution of unknown concentration was 0.161. Use your graphing calculator to generate a standard curve from this data set, and use it to determine the concentration of the unknown solution. Express the answer in micromolar (notice micro means 10-6)

Concentration of Blue Dye(M) Absorbance

@ 630 nm

7.58x10-6 0.101

3.26x10-5 0.288

5.17x10-5 0.521

8.63x10-5 0.782

Answer 1

According to Beer-Lambert's law:

Absorbance, A = c l, where

is molar extinction coefficient; c is the concentration of sample; l is the path length.

A plot of Absorbance(A) against concentration(c) gives a straight line with slope = molar extinction coefficient( ) and intercept at 0.

In order to find an unknown concentration from the above curve, just draw a straight line to the curve from y-axis (corresponding to A = 0.161) to the curve. The point at which it intercepts the curve, draw a straight line to x-axis. The point at which this line meets the x-axis will be the unknown concentration.

The concentration of the unknown = 14.5 M