UV\\VIS quiz 2 and answers

Quiz 2

18/02/14

If the wave length of a light is 2.5 micron, what is its corresponding wave number (cm -1)?

1. a.

4000

2.

b. 4050

c. 3000

d. 3050

What is the range of uv-visible radiation?

a.

200-700 mµ

b. 200-800 nm c.200-800µ

d.150-750nm

3. In which spectroscopic techniques the change in vibrational and rotational movements can be detected? a.

UV-Vis

4.

b. NMR

c. IR

d. MS

Which of the following is a forbidden transition?

a.

α-α*

b. π-π*

c. n- π*

d. Both a & b

5. In the past, IR spectra had to be aquired one wavelength at a time, which took a long time. Today quick spectra is due to the a. b. c. d.

the Fourier Transfer Algorithm allows us to scan all frequencies at once light is faster today that it used to be absence of broad spectrum of wavelength none of the above

6. What is spectroscopy? What is its importance?

Spectroscopy is the use of the absorption, emission, or scattering of electromagnetic radiation by matter to qualitatively or quantitatively study the matter or to study physical processes.

Spectroscopy is used in analytical chemistry for the identification of substances through the spectrum emitted from or absorbed by them.

7. Account for the fact that the n-π* absorption of acetone occurs at 279 nm in n-hexane, 270 nm in ethanol and at 265 nm in water.   

Peaks resulting from n-π* transitions are shifted to shorter wavelength(blue shift) with increasing solvent polarity Because of the increased solvation of the lone pair in the ground state Which lowers the energy of the n orbital

The ultraviolet spectrum of benzonitrile shows a primary absorption band at 224 nm and a secondary band at 271. a) If a solution of benzonitrile in water , with a concentration of 0.0001 molar, is examined at a wavelength of 224 nm, the absorbance is determined to be 1.30. The cell length is 1 cm. What is the molar absorptivity of this absorption band? Beer–Lambert law, A=Єbc Є=A/bc =1.30/(0.0001M*1cm) =13000M-1cm-1

b) If the same solution is examined at 271 nm, what will be the absorbance reading (Єmax = 1000)? Beer–Lambert law, A=Єbc =1000M-1cm-1*1cm*0.0001M =0.1 What will be the intensity ratio? Transmittance (T) is the intensity ratio A = -log T T = 10-A T = 10-0.1 T = 0.794

8. The UV spectrum of acetone shows absorption maximum at 166, 189 and 279 nm. a.

What type of transitions is responsible for each of these bands? 

166

n→

189

 →

279

n→

b.



Explain three applications each of UV-VIS and IR spectroscopy.

Lecture notes 9. What is the range of IR radiation? 10. Give four(4) comparison of the Dispersive IR and Fourier Transform IR. Lecture notes