Question

Pure hexane has negligible ultraviolet absorbance above a wavelength of 200 nm. A solution prepared by dissolving 25.8 mg of benzene (C6H6, FM 78.11) in hexane and diluting to 250.0 mL had an absorption peak at 256 nm with an absorbance of 0.266 in a 1.000-cm cell. Find the molar absorptivity of benzene at this wavelength.

Answer 1

Answer:

ε = 201.5 $M^{-1} cm^{-1}$

Explanation:

Given data-

mass of benzene = 25.8 mg

= 25.8×10^-3 kg

Atomic  mass of benzene= 78.11 amu

volume of hexane = 250.00 ml = 0.2500 L

Absorbance =  0.266

Length of the cell = 1.00 cm

Hence concentration = $\frac{25.8\times10^{-3}}{78.11\times0.25 }$

solving we get

= 0.00132 M

Formula:

A= εcl

A= absorbance

c= concentration and l=  length of cell

now substituting values in the above formula

0.266= ε×0.00132×1.000

⇒ε = $\frac{0.266}{0.00132\times1.000}$

⇒ε = 201.5 $M^{-1} cm^{-1}$

Answer 2

The molar absorptivity of benzene at this wavelength is $201.5M^{-1}cm^{-1}$

Beer-Lambert's Law:

It states that the amount of energy absorbed or transmitted by a solution is proportional to the solution's molar absorptivity and the concentration of solute. It is given as:

$A=E*l*c$

Given:

Mass of benzene = 25.8 mg = $25.8*10^{-3} kg$

Atomic  mass of benzene= 78.11 amu

Volume of hexane = 250.00 ml = 0.2500 L

Absorbance =  0.266

Length of the cell = 1.00 cm

To find:

Molar absorptivity =?

Calculation for concentration:

$\frac{2.8*10^{-3}}{78.11*0.25} =0.00132M$

On substituting the values in above equation:

$A=E*l*c\\\\0.266=E*1.000*0.00312\\\\E=201.5M^{-1}cm^{-1}$

Thus, the value for molar absorptivity of benzene at this wavelength is :$201.5M^{-1}cm^{-1}$.

Find more information about Beer-Lambert's law here:

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