# How do you solve for concentration knowing absorbance?

##### 1 Answer

Jun 8, 2016

You use Beer's law:

#\mathbf(A = epsilonbc),# where:

#A# is theabsorbanceof the solution. This isunitless.#epsilon# is theextinction coefficient, ormolar absorptivity, of thespecies in solutionin#"L"/("mol"cdot"cm")# .#b# is thepath lengthof the cuvet. This tends to be#"1 cm"# for typical UV-Vis spectrometry.#c# is theknown concentrationof your solution in#"M"# or#"mol/L"# .

Hence, all you really need to do is solve for

#color(blue)(c = (A)/(epsilonb))#

You already have

**If you have multiple absorbances, then you have multiple concentrations to solve for.**

You don't really need the wavelength to calculate concentration, but you should still report that. I'm guessing it is your

**Example:**

That is the wavelength where the light absorption generates the *tallest* absorbance signal.