Engineering Chemistry S Datta

INSTRUMENTAL METHODS OF ANALYSES 541
Q. 3. What are the parameters for expressing the absorption?
Ans. Transmittance is the ratio of the intensity of light transmitted to the intensity of
incident light.
where
(Transmittance) T = I T% (percent transmittance) = 100 × I
I o I
A = log I o
10
∴ A = 2 – log T%
I
A = Absorbance, I o
= Intensity of incident light,
I = Intensity of transmitted light.
· Combination of Lambert–Beer’s law given
A = A 1% 1cm bc.
A = Absorbance, A 1% 1cm
= Absorbance of 1% (w/v) solution for path length of 1 cm,
b = thickness (cm), c = concentration (g/100 ml)
A 1% × molecular wt.
1cm
ε (molar extinction coefficient) =
.
10
Q. 4. Calculate the concentration of a substance A in an ethanolic solution of
which the absorbance in a 1 cm cell at its λ max
241 nm was found to be 0.890. The A
(1%, 1 cm) is 540 at 241 nm.
Ans.
A = A 1% 1cm bc.
0.890 = 540 × 1 × c
∴ C = 0.00165 g/100 ml.
Q. 5. Calculate the concentration in µg ml –1 of a solution of substance B (mol.wt.
204.2) in 0.1 (M) HCl, showing absorbance at its λ max
277 nm of 0.613 in a 4 cm cell.
The molar absorptivity (ε) at 277 nm is 5432.
Ans. Here the working formula is A = εbc.
0.613 = 5432 × 4 × c
∴ c = 2.82 × 10 –5 mol l –1
= 2.82 ×10 –5 × 204.2 gl –1
= 0.00576 gl –1
= 5.76 µg ml –1 .
Q. 6. What is molecular spectroscopy?
Ans. The internal energy (E) of a molecule is given by:
E = Kinetic energy + Potential energy + E rot
+ E vib.
+ E trans
.
The molecular spectroscopy mainly deals with the study of interaction between radiant
energy and the molecule. In general molecular spectroscopy is absorption spectroscopy.
Q. 7. What are the different electronic transitions that take place on absorption
of UV light?
Ans. When a molecule, specially organic molecule, absorbs UV radiations the electrons
are excited to higher energy levels. In the diagram below the electrons are represented
R
H
n (nonbonding)
C=O:
:
o