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MCQ on UV-Visible spectroscopy: Page-8

1. During relaxation, the electron spin is reversed in

(A) Fluorescence

(B) Phosphorescence

(C) IR

(D) NMR

Elections are paired in an orbital with opposite spins according to Pauli’s exclusion principle. This rule is applied to the electrons when they are in both ground and excited state. But sometimes the electron in the excited state may undergo a spin change such that it has same spin as the electron in the ground state. This electron now termed as in triplet state jumps to ground state by releasing energy in the form of phosphorescence.

2. Which of the following is a real limitation to Beer’s law

(A) Fluorescence

(B) Analyte Dissociation

(C) Polychromatic radiation is used as source

(D) Analyte at high concentrations

Analyte when used at high concentration may result in non-linearity and deviation from Beer-Lambert’s law. This is mainly due to reduction of the speed of the light by the analyte compared to vacuum resulting in non-linearity.

3. Given below are the solvents used in analysis of benzaldehyde in the UV-visible spectra. Select the CORRECT statement regarding these
Solvent effect on benzaldehyde

(A) Water increases pi to pi transition

(B) Cylcohexane increases pi to pi transition

(C) Both water and cyclohexane increase pi to pi transition

(D) Water increases n to pi transition while cyclohexane increases pi to pi transition

Since the analyte is same in both samples, the relative differences in the absorption of the samples only depends on the solvent. Generally a polar solvent like water can show positive solvatochromism on pi to pi transition therefore water will increase pi to pi transition. Similarly, non-polar solvents like cyclohexane show positive solvatochromism on n to pi transition.

4. Related to the samples in above question, which of the following statements is more appropriate?

(A) Sample A shows more absorption than sample B

(B) Sample B shows more absorption than sample A

(C) Sample A shows more absorption while sample B shows no absorption

(D) Sample A shows equal absorption as sample B

As benzaldehyde mainly shows stronger pi to pi transition, the solvent which increases this transition results in more absorption. Since water is a polar solvent that shows positive solvatochromism on pi to pi transition, sample A will more absorption than sample B.

5. Dimerization of the sample produces

(A) Absence of absorbance

(B) Hypsochromic shift

(C) Bathochromic shift

(D) No effect