Interpretation of UV–Visible Spectrum and Calculation of λmax of Benzene

 

Experiment: Interpretation of UV–Visible Spectrum and Calculation of λmax of Benzene

Aim

To record and interpret the UV–Visible spectrum of benzene and determine its maximum absorption wavelength (λmax).

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Principle

Benzene contains a conjugated π-electron system. When ultraviolet light passes through benzene, the π-electrons absorb energy and undergo π → π* electronic transition.

The absorption of radiation occurs at specific wavelengths, producing peaks in the UV spectrum. The wavelength at which maximum absorption occurs is called λmax.

The absorbance follows Beer–Lambert Law:

$$A=\epsilon cl$$

Where

• A = Absorbance

• ε = Molar absorptivity

• c = Concentration

• l = Path length of the cuvette

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Apparatus

• UV–Visible spectrophotometer

• Quartz cuvette

• Volumetric flask

• Pipette

• Beaker

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Chemicals / Reagents

• Benzene sample

• Ethanol or cyclohexane (solvent)

• Distilled water

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Procedure

1. Prepare a dilute solution of benzene in ethanol or cyclohexane.

2. Fill the quartz cuvette with solvent and set it as blank in the UV–Visible spectrophotometer.

3. Fill another cuvette with the benzene solution.

4. Scan the spectrum in the range of 200–300 nm.

5. Record the absorbance values at different wavelengths.

6. Plot a graph of Absorbance vs Wavelength.

7. Identify the wavelength where the maximum absorbance occurs (λmax).

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Observation Table UV–Visible Spectrum of Benzene

Sr. No.	Wavelength (nm)	Absorbance
1	220	0.18
2	230	0.32
3	240	0.54
4	245	0.68
5	250	0.82
6	254	0.95
7	260	0.73
8	270	0.48

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Maximum absorbance observed at: 254 nm

$${\lambda }_{max}=254nm$$

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Calculation of λmax

λmax is determined from the peak of the absorbance curve.

$${\lambda }_{max}=\text{Wavelength at which absorbance is maximum}$$

For benzene, the characteristic absorption peak occurs at approximately:

$${\lambda }_{max}\approx 254\text{nm}$$

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Result

The UV–Visible spectrum of benzene was recorded and the maximum absorption wavelength (λmax) was found to be approximately 254 nm, corresponding to π → π* electronic transition in the aromatic ring.