How do you calculate concentration from absorbance?

The basic idea here is to use a graph plotting Absorbance vs. Concentration of known solutions. Once you have that you can compare the absorbance value of an unknown sample to figure out its concentration.

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You will be applying Beer's law to calculate the concentration.

The equation for Beer's law is: A = εmCl

(A=absorbance, εm = molar extinction coefficient, C = concentration, l=path length of 1 cm)

You should have a data set which was used to create a standard curve. The graph should plot concentration (independent variable) on the x-axis and absorption (dependent variable) on the y axis.

You'll need to add a line of best fit to the data points and determine the equation for the line. The equation should be in y=mx + b form.

y = absorbance (A)
Note: no unit for absorbance

x = concentration (C)
Note: unit is M or mol/L

m = (εm) = slope or the molar extinction coefficient in beers law which has units of #M^-1cm^-1#

So A = εmC +b

If you solve for C you should get
C = (A-b)/εm

So if you substract your y-intercept from the absorbance and divide by the slope, you are finding the concentration of your sample.

Here is video of a lab applying this concept.

Hope this helps!