The spectrum of a 1 mM solution of X and the spectrum of a 1 mM solution of Y we
ID: 1051982 • Letter: T
Question
The spectrum of a 1 mM solution of X and the spectrum of a 1 mM solution of Y were each recorded in 1 cm cells. When plotted on the same axes, the spectra cross each other at 1 = 400 nm. Use Beer's Law to demonstrate that the molar absorptivities of X and Y at 400 nm (epsilon_400, x andepsilon_400, y) must be equal. What is the Abs at 400 nm of a solution containing 0.2 M X and 0.3 M Y in a 1 cm cell, if the molar absorptivity (epsilon) is equal to 1, 500 M^-1 cm^-1. In the spectrum of Fe(phen)_3^2+, the lambda_max = 508 nm. On lecture slide 25 we calculated the energy of 508 nm light, and determined that this must be the energy gap between electronic levels in Fe(phen)_3^2+. However, the absorbance spectrum of Fe(phen)_3^2+ also shows significant absorbance slightly lower and slightly higher than 508 nm, from about 450-525 nm-in other words, the peak at 508 nm is not a single, skinny line at only 508 nm, but instead is broad and spans a range of wavelengths. What does this mean about the electronic states of the molecule, and why do you think we observe a broad peak?Explanation / Answer
1)a) A=ecl (Beer's law) where A=absorbance,e=molar absorptivity c=concentration of species in solution,l=path length=1.0 cm
as both X and have same concentration =1mM and same l=1.0 cm getting same absorbance at wavelength essentially means same value for e at 400nm
2)e=1500 M^-1 cm^-1
A=elc1+elc2=el(c1+c2) [absorption is additive]
or,A=(1500 M^-1 cm^-1) (1cm) (0.2M+0.3M)=750
2)Organic moecules bonds are not precise ,so electrons cannot jump from one precise energy level to next as in atoms .The bond has vibration so absorbs radiation at its resonant frequency and this is more prominent in conjugated system.Several overlapping peaks due to such minor absorptions appear as a broad peak with maximum absorption by any typical electron excitation ,to be treated as characteristics absorption of the compound
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