Organic Chemistry Multiple Choice Questions and Answers

1 The fact that the fluorescence wavelength is often much longer than the irradiation wavelength (Stokes shift) is a consequence of which phenomenon?

A low extinction coefficients (Lambert-Beer law)

B vertical transitions (Kasha’s rule)

C high ISC rates (El Sayed rule)

D the Franck–Condon principle

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Answer: the Franck–Condon principle

2 Which of the following is an incorrect statement?

A First step in photochemistry is excited state (photoexcitation)

B Photochemical reactions are caused by absorption of ultraviolet only

C When a molecule or atom in the ground state (S0) absorbs light, one electron is excited to a higher orbital level

D It is possible for the excited state S1 to undergo spin inversion

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Answer: Photochemical reactions are caused by absorption of ultraviolet only

3 Which regions of the light radiations of the visible ultraviolet lying between – wavelength are chiefly concerned in bringing about photochemical reactions?

A 1000 Å and 2000 Å

B 1500 Å and 1000 Å

C 8000 Å and 2000 Å

D 19000 Å and 12,000 Å

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Answer: 8000 Å and 2000 Å

4 Which of the following instruments is used to measure the energy of the monochromatic radiation most accurately?

A Photoelectric cell

B Thermopile

C The potential detector

D The chemical actinometer

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Answer: Thermopile

5 The molar extinction coefficient of B (MW = 180) is 4 x 10³ L mol^-1 cm^-1. One-liter solution of C which contains 0.1358 g pharmaceutical preparation of B, shows an absorbance of 0.411 in a 1 cm quartz cell. What is the percentage (w/w) of B in the pharmaceutical preparation?

A 10.20

B 13.60

C 20.40

D 29.12

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Answer: 13.60

6 What are the appropriate reasons for the deviation from the Beer’s law among the following?

(A) Monochromaticity of light

(B) Very high concentration of analyte

(C) Association of analyte

(D) Dissociation of analyte

A A, B and D

B B, C and D

C A, C and D

D A, B and C

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Answer: B, C and D

7 A 0.1 M solution of compound A shows 50% transmittance when a cell of 1 cm width is used at λ₁ nm. Another 0.1 M solution of compound B gives the optical density value of 0.1761 using 1cm cell at λ₁ nm. What will be the transmittance of a solution that is simultaneously 0.1 M in A and 0.1 M in B using the same cell and at the same wave length? [log 1.301; log 1.4771; log 50 = 1.699].

A 33.3%

B 50%

C 66.7%

D 70%

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