Question

Calculate the extinction coeffiction (ε, absorptivity) of each compound using Beer-Lambert's Law ; A = εbc. Because of the small amount of material used, you get 2 significant figures.

A = absorbance

ε = absorptivity

b = path length (1 cm)

c = concentration (moles/L)

My masses were 1.9mg (1), 2.3mg (2) and 2.1mg (3). The MW were 572.53 (1) , 486.35 (2) and 512.39 (3) . The max wavelength were 483.5nm = 0.452 (1) , 583 nm = 0.772 (2) and 687.4nm = 0.736 (3). Please show how to get concentrations as well as the extinction coefficient.

Answer 1

The concentration (c) is expressed in mole/litre.

Concentration in moles/litre (Molarity) = (mass / MW x volume in litre) and Molar extinction coefficient is calculated from   Absorbance (A) = ε b c

For sample 1: Molarity = (1.9 mg / 572.53 g x 1litre) = (0.0019 g / 572.53 g x 1 litre) = 3.3 x 10^-6 M

Absorbance (A) = ε b c , then ε =A/bc = (0.452/1x3.3 x 10^-6) = 0.136 x 10⁶ =1.36 x 10⁵ L Mol-1 cm-1

For sample 2: Molarity = (2.3 mg / 486.35 g x 1litre) = (0.0023 g / 486.35 g x 1 litre) = 4.7 x 10^-6 M

Absorbance (A) = ε b c , then ε =A/bc = (0.772/1x4.7 x 10^-6) = 0.164 x 10⁶ =1.64 x 10⁵ L Mol-1 cm-1

For sample 2: Molarity = (2.1 mg / 512.39 g x 1litre) = (0.0021 g / 512.39 g x 1 litre) = 4.09 x 10^-6 M

Absorbance (A) = ε b c , then ε =A/bc = (0.736/1x4.09 x 10^-6) = 0.179 x 10⁶ =1.79 x 10⁵ L Mol-1 cm-1