Question

please answer 1-4 &6

Questions The energy of UV light is 70 kcal/mole at 400 nm and 140 kcal/mole at 200 nm. Why is UV radiation damaging to the eyes and skin? (Hint: Think about the energy required to break a C-C bond.) 2. Which is more energetic: 235 nm or 325 nm light? Explain. 3.) Why is UV spectroscopy used more frequently in organic chemistry than visible spectroscopy? 4. Using the structure of methyl orange as an example, draw resonance forms to show how conjugation extends throughout the molecule. S. Depict the molecular orbitals for 1,3,5-hexatriene. Show the electron occupancy of each orbital. Label the LUMO and HOMO. Is the expected wavelength for UV excitation at a longer or shorter wavelength than that for 1,3-butadiene? Calculate the molar absorptivity (e) of benzene at 260 nm for a solution of benzene in ethanol where the concentration of benzene is 3.3 x 10" M and the absorbance reading is 0.70 using a 1-cm cell. T antum angu

Answer 1

1) The C-C bond energy is around 80 kcal/mol. Hence using this energy C-C bond can be broken. This much amount of energy or higher is possible to gain by 200 nm UV light. Thus upon exposure, the certain C-C bond present in the eyes or skin molecules are raptured causing damage to the eyes or skin.

2) Energy is inversely proportional to the wavelength by the equation:

E = h · c / λ  

Thus 235 nm light is more energetic than the 325 nm light.

3) In organic chemistry, we deal with more often the chromophore and auxochrome molecules that have strong absorption in the UV region. e.g. Benzoic acid is colorless and does not have much absorption in the visible light range. However, due to the presence of conjugation and non-bonding electrons it has strong absorption in UV range.

Hence we use UV spectroscopy more frequently than the visible spectroscopy.

4)

དཔེར་ན་ རྒྱ ལ ་ ་ This is a red form of methyl orange.

6) As per the Beer-Lambert law:

Absorbance (A) = $\varepsilon$ *C* l where $\varepsilon$ = molar absorptivity, C=concentration of benzene,   l = pathlength

$\varepsilon$  =  A / (c*l) = 0.70 / ((3.3*10^-3)*1) = 212.12 M^-1cm^-1