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Beer-Lambert Law Calculator

Absorbance related to concentration.

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Absorbance

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Overview

The Beer-Lambert Law defines the linear relationship between the absorbance of a substance and its concentration in a solution. It posits that as light passes through a medium, the intensity of light absorbed depends on the chemical properties of the solute, the distance the light travels, and the molar density of the sample.

Symbols

Variables

A = Absorbance, = Molar Absorptivity, l = Path Length, c = Concentration

Absorbance
Variable
Molar Absorptivity
L/mol cm
Path Length
cm
Concentration
mol/L

Apply it well

When To Use

When to use: Use this equation when performing spectrophotometry to determine the concentration of a known solute in a solution. It assumes monochromatic light is used, the solution is dilute (typically below 0.01 M), and there are no chemical fluctuations or light scattering within the sample.

Why it matters: It is the foundational principle for modern chemical analysis, enabling everything from monitoring pollutants in water to quantifying DNA or proteins in biological research. Its simplicity allows for rapid, non-destructive testing in pharmaceutical and industrial quality control.

Avoid these traps

Common Mistakes

  • Forgetting path length l.
  • Confusing absorbance with transmittance.

One free problem

Practice Problem

A chemical dye with a molar absorptivity of 5000 M⁻¹cm⁻¹ is analyzed in a spectrophotometer. If the concentration of the solution is 0.0002 M and the path length of the cuvette is 1.0 cm, what is the measured absorbance?

Molar Absorptivity5000 L/mol cm
Path Length1 cm
Concentration0.0002 mol/L

Solve for:

Hint: Multiply the molar absorptivity, path length, and concentration together (e × l × c).

The full worked solution stays in the interactive walkthrough.

References

Sources

  1. Atkins' Physical Chemistry
  2. Wikipedia: Beer-Lambert law
  3. IUPAC Gold Book: Beer-Lambert law
  4. Atkins' Physical Chemistry, 11th ed.
  5. Principles of Instrumental Analysis, Skoog, Holler, Crouch, 7th ed.
  6. Skoog, D. A., Holler, F. J., & Crouch, S. R. (2017). Principles of Instrumental Analysis (7th ed.). Cengage Learning.
  7. Atkins, P., & de Paula, J. (2014). Atkins' Physical Chemistry (10th ed.). Oxford University Press.
  8. IUPAC Gold Book (Compendium of Chemical Terminology).