Absorbance depends on the thickness of the sample size. The cuvets for most spec
ID: 492949 • Letter: A
Question
Absorbance depends on the thickness of the sample size. The cuvets for most spectrophotometers hold the sample size to 1 cm, although they can vary. The molar absorptivity (epsilon) is equal to the absorbance of a one-centimeter thick sample of a 1.0 M solution. In this case, epsilon has the units L mol^-1 cm^-1. Epsi changes with wavelength and is generally given for the wavelengths corresponding to the maxima in the absorption curve. How is the constant you found related to the molar absorptivity? A solution known to contain 0.026 M Cu^2+ forms a complex with ethylenediamine that yields an absorbance of 0.37 in a cell of 1.0 cm path length at a selected wavelength. Calculate elementof for these conditions.Explanation / Answer
Molar absorptivity, as stated, can be related to different concentrations
A typically depends on:
A = e*l*C
where
e = molar absorptivity (Lmol^-1cm^-1)
l = length of sample i.e 1 cm
C = molar concentration ( mol/L)
so..
an increase in A will favour an increase
so
A = e*l*C
can be assumed to be linear and constant as follows:
A = X*C
where X = e*l constant value
And this is the wqay we relate absorbption and concentration
Q3.
M = 0.026 M of Cu+2
A = 0.37
l = 1 cm
for this vlaue, calculate e:
from the formula previously stated
A = l*e*C
substitute all values
note that A = unitless
0.7 = e* (1 cm) * (0.026 mol/L)
so
e = 0.7 / (1 cm * 0.026 mol) / L = 1.8918 L/mol-cm
e = 1.8918 L/mol-cm