Chlorophyll a is approximately plant whose size is about 4nm^2. Its molar mass i

Question

Chlorophyll a is approximately plant whose size is about 4nm^2. Its molar mass is 902.5 g/mol and it absorbs strongly at wavelength= 660nm (red light). When light of this wavelength was passed through a 1.0cm cuvette containing chlorophyll of concentration 11 ug/cm^3, it was found that only 12.5% of the incident light was transmitted.

a) determine the concentration of chlorophyll a in mol/L 
b) Determine the molar extinction coefficient for chlorophyll a (in units of mol/L cm) 
c) determine the absorption cross-section of the molecule for wavelength= 660nm. How many times is the absorption cross-section of chlorophyll smaller than its real size?

Explanation / Answer

A. C = 11 ug/cm^3 / 902.5 g/mol

        = 11 * 10-6 / 1 * 10-3 * 902

        =0 .012 * 10-3 mol / l = 12 * 10-6 mol/l

B. A = log 1/T

       = log (100/12.5) = log 8 = 0.90

   A = ecl

   0.90 = e * 12 * 10-6 mol /l * 1 cm

e = 0.90/12 * 10-6 l/mol cm

e = 0.075 * 106 = 75 * 103 l/mol cm

C. A = * L * N

- absorption cross section cm2/molec L = absorption path length [cm] N = density of the absorber [molec/cm3]

N = 11 * 10-6 g / 902.5 g / mol = 0.012 * 10-6 mol

0.012 * 10-6 mol * 6.023 * 1023 = 7.2 * 1015 molecule/cm3

0.90 = * 1 * 7.2 * 1015 molecule

= 0.125 * 10-15 = 125 * 10-18 cm2/molecule.

125 * 10-18 / 4 10-18

31 times smaller than real values

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