Mass from Absorbance

Question

The molar absorptivity, ε, for the dye called 2′,7′-dichlorofluorescein (C₂₀H₁₀Cl₂O₅, molar mass = 401.201 g mol^-1) was found by experiment to be 5.84×10⁵ L mol^-1 cm^-1 at 509 nm. The percent transmittance of a 585 mL solution of 2′,7′-dichlorofluorescein was determined to be 90.3%. If the path length of the cell is 1.01 cm, determine the mass (in g) of 2′,7′-dichlorofluorescein present in the solution. Report your answer to 3 significant figures in scientific notation.

Answer 1

Absorbance = 2-log %T

A = 2-log 90.3 = 2 - 1.9557

A = 0.0443

From Beers Law, A = ecl

0.0443 = 5.84x10⁵ * c * 1.01

c = 7.51x10^-2 Moles/L

7.5x10^-2 mol/L x 0.585 L x 401.2 g/mol = 17.6 grams