   Chapter 6, Problem 72IL

Chapter
Section
Textbook Problem

The spectrum shown here is for aspirin. The vertical axis is the amount of light absorbed, and the horizontal axis is the wavelength of incident light (in nm). (For more on spectrophotometry, see Section 4.9) What is the frequency of light with a wavelength of 278 nm? What is the energy of one mole of photons with ʎ = 278 nm? What region of the electromagnetic spectrum is covered by the spectrum above? Knowing that aspirin only absorbs light in the region depicted by this spectrum. What is the color of aspirin?

Interpretation Introduction

Interpretation: The frequency and energy per mole of photons has to be calculated. The region of absorption spectrum and colour of aspirin has to be identified.

Concept introduction:

• Electromagnetic radiations are a type of energy surrounding us. They are of different types like radio waves, IR, UV, X-ray etc.
• Visible light lies in the wavelength of 400nm to 700nm • Planck’s equation,

E=where, E=energyh=Planck'sconstantν=frequency

The energy increases as the wavelength of the light decrease. Also the energy increases as the frequency of the light increases.

• The frequency of the light is inversely proportional to its wavelength.

ν=cλwhere, c=speedoflightν=frequencyλ=wavelength

• Electromagnetic radiations are a type of energy surrounding us. They are of different types like radio waves, IR, UV, X-ray etc.
• Ultraviolet radiation lies between 200nm to 400nm wavelength region.
Explanation

Given,

The wavelength of light is 278 nm=2.78 ×107m.

• The frequency per mole of photons is calculated,

ν=cλ

Substituting the values

ν=2.998×108m/s2.78 ×107m=1.078×1015s1

The frequency per mole of photons is 1.078×1015s1

• The energy per mole of photons is calculated.

Energyperphoton==6

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