The molar absorption coefficient of a substance dissolved in benzene is measured

Question

The molar absorption coefficient of a substance dissolved in benzene is measured to be 784 L mol^-1 cm^-1 at 259 nm. Calculate what fraction of the incident light intensity of that wavelength passes through 2.31 mm of a solution of concentration 1.83 mmol/L. A solution in an absorption cell of path length 2.00 cm transmits 40.0 per cent of light of 339 nm incident upon it. If the concentration of the component is 0.400 mmol/L, what is the molar absorption coefficient at this wavelength? L mol^-1 cm^-1.

Explanation / Answer

1) Given e= 784. l = 2.31mm = 0.231cm concentrationc = 1.83mmol/L = 1.83x10-3M

Thus absorbance can be calculated using Beer-Lambert law

      A = log [Io/I] = e.l.c.

A= 784x0.231x1.83x 10-3 = 0.331 and I/Io = 0.466

   thus the fraction of incident light that passes through solutionis 0.466

2. given l = 2 cm , I/Io = 0.4 ,   C = 0.4mmol/L = 4x 10-4 M

Thus A = log 1/0.4 = e x 2x4 x10-4

and e= 497.4 L mol-1 cm-1

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