Question

A 0.0000831 M sample of Compound X in a solvent has an absorbance of 0.349 at 528 nm in a 1.000-cm cuvet. The solvent alone has an absorbance of 0.056. What is the molar absorptivity of Compound X?

A different sample of Compound X in the same solvent has an absorbance of 0.501 at 528 nm when measured with the same cuvet. What is the concentration of this second sample?

Answer 1

According to Beer Lamberts law

absorbance = molar absorptivity coefficient x path length of the cuvette x concentration of the sample

Given that concentration =  0.0000831 M

path length of the cuvette = 1 cm

absorbance at 528 nm = 0.349 and contribution from the solvent is 0.056

contribution from sample = 0.349-0.056 = 0.293

thus ,

molar absorptivity coefficient at 528 nm= 0.293/0.0000831 = 3525.872 M-1cm-1

Case 2

Absorbance of X = 0.501-0.056 = 0.445 (Note: substracting solvent absorption)

we know molar absorptivity coefficient of X at 528 nm3525.872 M-1cm-1

on applying Beer Lamberts law

Concentration = absorbance / (molar absorptivity coefficient x path length) = 0.445/3525.872 = 0.000126 M

note: path length of cuvette is 1 cm