You want to calculate the concentration, c, of a solution of nitrobenzene in hex

Question

You want to calculate the concentration, c, of a solution of nitrobenzene in hexanes. At 252 nm, the maximum wavelength of absorbance, lambda_max, for nitrobenzene, it's molar absorptivity, epsilon, is 1.0 times 10^5 M^-1 cm^-1. You take a UV-Visible spectrum of the solution using a 1 cm wide cuvet, and find that at 252 nm the absorbance is 1.2. (Adapted from Organic Chemistry by Paula Bruice.) Using Beer's law, calculate the concentration of your solution. ("1" is the length of the cuvet) Beer's Law: A = epsilon middot c middot l Would you be able to measure an accurate concentration if the solution were in benzene rather than hexanes? Explain your answer. (Assume that epsilon for the two solvents is the same.) Would you be able to measure the concentration of your solution if you measure the absorbance at 242 nm? Explain your answer.

Explanation / Answer

a)   Given that absorbance A = 1.2

lenth of cuvette l = 1 cm

molar absorptivity of nirobenzene , e = 1.0 x 105 M–1cm–1

concentration of nirobenzene solution, c = ?

From Beer's law,

A = ecl

c = A / el

   = (1.2)/ (1.0 x 105 M–1cm–1) (1 cm)

   = 0.000012 M

c = 0.000012 M

Therefore,

concentration of nirobenzene solution = 0.000012 M

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