# How do you calculate concentration from absorbance?

You will use Beer’s law. A = εmCl

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.

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!

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