pery
A CaCO3 solution shows a transmittance of 90% when
taken in a cell of 1.9 cm thickness. Calculate it's
concentration, if the molar absorption coefficient is 9000
dm3/mol/cm
UDA, AT
E
GROWTH
PPLIED BC
3.6 x10-5
mol/dm3
MISTRY)-
MISTRY)-T
LIAN PHY
2.63 x10-6
mol/dm3
1.62 x10-6
mol/dm3
GY-II)-TH
r.A.C
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Answers
Answer:
The Beer-Lambert law relates the attenuation of light to the properties of the material through which the light is traveling. This page takes a brief look at the Beer-Lambert Law and explains the use of the terms absorbance and molar absorptivity relating to UV-visible absorption spectrometry.
The Absorbance of a Solution
For each wavelength of light passing through the spectrometer, the intensity of the light passing through the reference cell is measured. This is usually referred to as Io - that's I for Intensity.
cuvette
The intensity of the light passing through the sample cell is also measured for that wavelength - given the symbol, I . If I is less than Io , then the sample has absorbed some of the light (neglecting reflection of light off the cuvette surface). A simple bit of math is then done in the computer to convert this into something called the absorbance of the sample - given the symbol, A . The absorbance of a transition depends on two external assumptions.
The absorbance is directly proportional to the concentration ( c ) of the solution of the sample used in the experiment. hi
The absorbance is directly proportional to the length of the light path ( l ), which is equal to the width of the cuvette.
Assumption one relates the absorbance to concentration and can be expressed as
A∝c(1)
The absorbance ( A ) is defined via the incident intensity Io and transmitted intensity I by
A=log10(IoI)(2)
Assumption two can be expressed as
A∝l(3)
Combining Equations 1 and 3 :
A∝cl(4)
This proportionality can be converted into an equality by including a proportionality constant ( ϵ ).
A=ϵcl(5)
This formula is the common form of the Beer-Lambert Law, although it can be also written in terms of intensities:
A=log10(IoI)=ϵlc(6)
The constant ϵ is called molar absorptivity or molar extinction coefficient and is a measure of the probability of the electronic transition. On most of the diagrams you will come across, the absorbance ranges from 0 to 1, but it can go higher than that. An absorbance of 0 at some wavelength means that no light of that particular wavelength has been absorbed. The intensities of the sample and reference beam are both the same, so the ratio Io/I is 1 and the log10 of 1 is zero.
Example 1
In a sample with an absorbance of 1 at a specific wavelength, what is the relative amount of light that was absorbed by the sample?
Solution
This question does not need Beer-Lambert Law (Equation 5 ) to solve, but only the definition of absorbance (Equation 2 )
A=log10(IoI)
The relative loss of intensity is
I−IoIo=1−IIo
Equation 2 can be rearranged using the properties of logarithms to solved for the relative loss of intensity:
10A=IoI
10−A=IIo
1−10−A=1−IIo
Substituting in A=1
1−IIo=1−10−1=1−110=0.9
Hence 90% of the light at that wavelength has been absorbed and that the transmitted intensity is 10% of the incident intensity. To confirm, substituting these values into Equation 2 to get the absorbance back:
IoI=10010=10(7)
and
log1010=1(8)
The Beer-Lambert Law
You will find that various different symbols are given for some of the terms in the equation - particularly for the concentration and the solution length
Answer:
According to beer Lamberts law :
Each layer of equal thickness of the medium absorbs an equal fraction of the energy traversing it.
using the given data ,
We have,
s = A / l c
l= 0.5 cm
A= 0.54
= 6.4 103
LMol-1
cm-1
C=?
So c = A/ l
= 0.54 / 6.4 103 0.5
Answer = 0.000168 M
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