Case 1
- Molar absorptivity
- 18400L/(mol·cm)
- Concentration
- 0.0001mol/L
- Path length
- 1cm
1.84
Open with these valuesBeer-Lambert Law Calculator
1.84
Result: 1.84Absorbance is the product of three numbers: how strongly the substance absorbs, how much of it is dissolved, and how far the light travels through it. Double any one of them and the absorbance doubles. The result has no unit, provided the inputs use L/(mol·cm), mol/L and cm.
The numbers at a glance
Held fixed: Molar absorptivity 18,400.00 L/(mol·cm), Concentration 0.000100 mol/L.
| Path length (cm) | Result |
|---|---|
| 0.25 | 0.46 |
| 0.50 | 0.92 |
| 0.75 | 1.38 |
| 1.00Your value | 1.84 |
| 1.25 | 2.30 |
| 1.50 | 2.76 |
| 1.75 | 3.22 |
| 2.00 | 3.68 |
Worked examples
Case 2
- Molar absorptivity
- 5000L/(mol·cm)
- Concentration
- 0.001mol/L
- Path length
- 0.5cm
2.50
Open with these valuesCase 3
- Molar absorptivity
- 220000L/(mol·cm)
- Concentration
- 0.00001mol/L
- Path length
- 1cm
2.20
Open with these valuesHow it's calculated
A = ε × c × l
- StepEnter the molar absorptivity of the substance at your wavelength.
- StepEnter the concentration of the solution in mol/L.
- StepEnter the path length of the cuvette in centimetres.
- ResultRead the absorbance — a pure number, no unit.
Reference table
| ε, c, l | What it stands for | Absorbance |
|---|---|---|
| 1, 1, 1 | The bare product, nothing else | 1 |
| 5000, 0.001, 0.5 | Half-width cuvette | 2.5 |
| 18400, 0.0001, 1 | A standard 1 cm cuvette | 1.84 |
| 18400, 0.0002, 1 | Twice the concentration | 3.68 |
| 220000, 0.00001, 1 | A strong absorber, very dilute | 2.2 |
Questions
What does absorbance mean?
It is the base-ten logarithm of how much light goes in divided by how much comes out. An absorbance of 1 means a tenth of the light survives, 2 means a hundredth.
Why does the answer have no unit?
The three units cancel: L/(mol·cm) times mol/L times cm leaves nothing behind. That only works if the inputs really use those units.
Up to which absorbance is a reading trustworthy?
Roughly 2, sometimes 3 on a good instrument. Beyond that so little light reaches the detector that the reading drowns in noise, even though the arithmetic keeps working.
Can I get the concentration back out of it?
Yes — divide the absorbance by the molar absorptivity and the path length. That is the everyday use of the law in a laboratory.
Where does molar absorptivity come from?
It belongs to the substance and the wavelength, and it is measured, not calculated. Tables and the source behind this calculator carry values for common compounds.
Sources and last check
Information, not professional advice.