# Pa III (Pr) CA II

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Uv Visible Spectroscopy

• 1.

### Beer’s Law (or the Beer-Lambert Law) is . . .

• A.

A linear relationship between the intensity of a UV absorbance and the concentration of the analyte.

• B.

An inverse relationship between the IR stretching frequency and the energy of light.

• C.

An inverse relationship between the IR stretching frequency and the energy of light.

• D.

Used to derive a molecular formula from the mass-to-charge ratio of an analyte.

A. A linear relationship between the intensity of a UV absorbance and the concentration of the analyte.
Explanation
Beer's Law, also known as the Beer-Lambert Law, states that there is a linear relationship between the intensity of a UV absorbance and the concentration of the analyte. This means that as the concentration of the analyte increases, the absorbance of UV light also increases in a proportional manner. This relationship allows scientists to quantitatively measure the concentration of a substance by measuring its absorbance of UV light.

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• 2.

### Which of the following wavelength ranges is associated with UV spectroscopy?

• A.

0.8 - 500µm

• B.

400 - 100nm

• C.

380 - 750nm

• D.

0.01 - 10nm

B. 400 - 100nm
Explanation
UV spectroscopy is a technique that involves the measurement of the absorption or emission of ultraviolet (UV) light by molecules. The wavelength range associated with UV spectroscopy is typically between 400 and 100 nanometers (nm). This range is chosen because it corresponds to the energy levels of electrons in molecules, allowing for the study of electronic transitions. The other wavelength ranges listed in the options, such as 0.8 - 500µm, 380 - 750nm, and 0.01 - 10nm, do not fall within the UV range and are associated with other spectroscopic techniques.

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• 3.

### Which of the following compounds does not absorb light in the UV/visible spectrum?

• A.

Aspirin

• B.

Paracetamol

• C.

Chloral hydrate

• D.

pHenobarbitone

C. Chloral hydrate
Explanation
Chloral hydrate does not absorb light in the UV/visible spectrum because it lacks any chromophores or conjugated systems that are responsible for absorbing light in that range. Chromophores are groups of atoms within a molecule that have delocalized electrons, allowing them to interact with light and absorb specific wavelengths. Aspirin, Paracetamol, and Phenobarbitone all contain chromophores or conjugated systems, which enable them to absorb light in the UV/visible spectrum. Therefore, Chloral hydrate is the compound that does not absorb light in this range.

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• 4.

### In UV absorption spectroscopy, which of the following statements are true?

• A.

σ→σ* transitions are lower in energy than π→π* transitions

• B.

A conjugated systems of double bonds in a molecular shifts the a absorption maxima to higher wavelengths

• C.

σ→σ* transitions are higher in energy than π→π* transitions

• D.

B and C

B. A conjugated systems of double bonds in a molecular shifts the a absorption maxima to higher wavelengths
Explanation
Conjugated systems of double bonds in a molecule shift the absorption maxima to higher wavelengths. This is because the delocalization of electrons in conjugated systems allows for the formation of lower energy molecular orbitals, resulting in longer wavelength absorptions. This phenomenon is commonly observed in UV absorption spectroscopy.

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• 5.

### A compound X is characterized in its electronic spectrum by an absorption with λmax = 217 nm (εmax = 21 000 dm3 mol–1 cm–1). Of the compounds given below, X is most likely to be:

• A.

Buta-1,3-diene

• B.

β-carotene.

• C.

Ethanol.

• D.

Water.

A. Buta-1,3-diene
Explanation
The correct answer is buta-1,3-diene. This is because buta-1,3-diene is a conjugated system of double bonds, which is known to exhibit strong absorption in the UV-Vis region. The absorption at λmax = 217 nm with εmax = 21,000 dm3 mol-1 cm-1 indicates a highly conjugated system, which is characteristic of buta-1,3-diene. β-carotene is also a conjugated system, but its absorption maximum is typically around 450-550 nm, not 217 nm. Ethanol and water do not have conjugated systems and would not exhibit such strong absorption in the UV-Vis region.

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• Current Version
• Mar 14, 2023
Quiz Edited by
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• Oct 10, 2018
Quiz Created by
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