Question

So, we did a Beer's Law lab and I am trying to find out how to find the concentrations to make the absorbace vs concentration graph. Say the path length is 1.0 cm, how do you find the molar absorbtivity (in order to get the concentrations with A = epsilon * path length * concentration)? I think there is a way to get the molar absorbtivity from the wavelength vs absorbance graph but I'm not sure how.

Our data is below for the wavelength vs absorbance:

400 - 0.46

450 - 0.73

500 - 1.01

550 - 1.09

600 - 0.78

650 - 0.54

700 - 0.23

Answer 1

Solution: Beer;s law is written mathematically as,

A = ɛ c l

where A= absorbance, ɛ= molar absorbitivity or molar extinction coefficient, c=concentration and l= path length.

Calculation for Concentration :Concentration of the sample is determined very easily during its preparation by molarity relation, M = (weight of sample/ molecular weight of sample x volume in litre)

M= moles per litre =c

Hence, in this way one can easily prepared the different samples having different concentration.

Then place the sample in Spectrophotometer and find the absorbance, A. Since path length, l is known (1 cm).

By knowing A, c and l, one can easily calculate the value of  ɛ, as

ɛ = A / cl

Concentration effects the absorbance very similarly to path length. If the concentration of solution is increased, then there are more molecules for the light to hit when it passes through. As the concentration increases, there are more molecules in the solution and causes the solution to get darker because less light can get through.

For every measurement of A, you can get a corresponding wavelength value which gives an indication about the range of the absorption, Higher wavelength confirms bathochromic shift (Red shift) and lower value of wavelength indicates hypsochromic shift (blue shift).