Information: The Beer-Lambert law allows you to measure the concentration of a s
ID: 1008946 • Letter: I
Question
Information:
The Beer-Lambert law allows you to measure the concentration of a solution based on its absorbance of light at a specific wavelength. This law is: A = elc
In this equation A is the absorbance (no unit) When c (the concentration of the solution) is expressed in mole/Liter (mol/L or mol.L-1 ) and l (the length of the light path) is expressed in cm, the e (the molar absorption coefficient) is expressed in Liter/(Mole x centimeter) or L.mol-1 .cm- 1
In fact, e can also be expressed in cm2 .mol-1 and m2 .mol-1 . In these cases, the units used for the concentration have to change: liters are replaced by cm3 or m3 , and l in some cases has to be expressed in meter (m) and not centimeter (cm).
Question:
1) A solution of tryptophan with a concentration c = 30 mg.L-1 has an absorbance A= 0.794 when the length of the light path is l= 1cm. Knowing that the molecular mass of tryptophan is 204 g.mol-1 calculate the molar absorption coefficient of the tryptophan solution expressed in L.mol-1 .cm-1 and in m2 .mol-1 . Show your calculations.
Explanation / Answer
A = 0.794
C = 30 mg / L = 30 x 10^-3 g / L
C = 30 x 10^-3 / 204 = 1.47 x 10^-4 mol / L
path length b = 1 cm
A = E b C
0.794 = E x 1 x 1.47 x 10^-4
E = 5399.2 L.mol-1 .cm-1
molar absorption coefficient = 5399.2 L.mol-1. cm-1