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MCQ on UV-Visible spectroscopy: Page-9

Molar extinction coefficient has units of

(A) lit.mol-1.cm-1

(B) dl.g -1cm -1

(C) No units

(D) lit.g -1cm -1

2. Two samples each containing same analyte at equal concentration are irradiated with different intensities of radiation as shown below. Select the CORRECT statement regarding the absorption shown by two samples.
Source with different intensities

(A) A 2=4 A 1

(B) A 2=2 A 1

(C) A 2=A 1

(D) A 2=1/2 A 1

Absorbance is independent of intensity of the incident radiation as it is the ratio between intensities of incident radiation (I o) and transmitted radiations (I t). If you double I o, then I t will also be doubled proportionally making absorbance remains constant.

3. Which of the following statement is not correct ?

(A) Absorptivity changes with the intensity of the light

(B) Absorbance is independent of intensity of the light

(C) Absorbance has no units

(D) Absorptivity is a constant and depends on the nature of the molecule

Absorptivity is the intrinsic property of a molecule and it will be a constant at a specific temperature and wavelength of radiation. So it doesn’t change with intensity of incident radiation at a particular wavelength. that

4. The possible transitions in the following compounds are
amine, alcohols, halides

(A) n→π*, π→π*

(B) σ→σ*, n→π*, π→π*

(C) σ→σ*, n→σ*

(D) n→π*, n→σ*

All the listed compounds contain similar type of electrons such as sigma and n electrons. Therefore they can produce two types of transitions such as s→s* and n→s*. Both these transitions fall below 200 nm therefore show absorption only in vacuum UV region. These compounds can’t show p→p* and n→p* as they don’t have pi electrons.

5. The above listed compounds are transparent in

(A) Vacuum UV region

(B) UV-visible region

(C) Visible only

(D) UV only

Since they doesn’t absorb in UV-visible region, they are transparent and colourless in this region. For a compound to absorb in UV-Visible region, it should have pi electrons.