Question

A 3.96x10^-24 M solution of compound A exhibited an absorbance of 0.624 at 238 nm in a 1.000-cm cuvet; a blank solution containing only solvent had an absorbance of 0.029 at the same wavelength. Find the molar absorptivity of compound A.

Answer 1

Actual question from source:-

A 3.96x10-4 M solution of compound A exhibited an absorbance of 0.624 at 238 nm in a 1.000 cm cuvette.  A blank had an absorbance of 0.029.  The absorbance of an unknown solution of compound A was 0.375.  Find the concentration of A in the unknown.

Answer:

Molar absorptivity of compound A = $1502.53\ {Ms}^{-1}$

Explanation:

According to the Lambert's Beer law:-

$A=\epsilon l c$

Where, A is the absorbance

 l is the path length  

$\epsilon$ is the molar absorptivity

c is the concentration.  

Given that:-

c = $3.96\times 10^{-4}\ M$

Path length = 1.000 cm

Absorbance observed = 0.624

Absorbance blank = 0.029

A = 0.624 - 0.029 = 0.595

So, applying the values in the Lambert Beer's law as shown below:-

$0.595=\epsilon\times 1.000\ cm\times 3.96\times 10^{-4}\ M$

$\epsilon=\frac{0.595}{3.96\times 10^{-4}}\ {Ms}^{-1}=1502.53\ {Ms}^{-1}$

Molar absorptivity of compound A = $1502.53\ {Ms}^{-1}$

Answer 2

The molar absorptivity of the sample is 1503 M-1cm-1.

Using the Beer - Lambert's law;

A = εcl

A = Absorption

ε = molar absorptivity

c = concentration

l = path length

The actual absorbance of the solute is obtained by subtracting the absorbance of the blank from that of the solution ;  0.624 - 0.029 = 0.595

ε = A/cl

A = 0.595

l = 1.000-cm

c = 3.96x10^-4 M

ε =  0.595/3.96x10^-4 M × 1.000-cm

ε = 1503 M-1cm-1

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