Iodine reacts with mesitylene to form a complex with an absorption maximum at 332 nm in CCl₄ solution:

(a) Given that the product absorbs at 332 nm, but neither reactant has significant absorbance at this wavelength, use the equilibrium constant, K, and Beer's law to show that

where A is the absorbance at 332 nm, ε is the molar absorptivity of the complex at 332 nm, [mesitylene] is the concentration of free mesitylene, and [I₂]_tot is the total concentration of iodine in the solution (= [I₂] + [complex]). The cell pathlength is 1.000 cm.

(b) Spectrophotometric data for this reaction are shown in the table. Because [mesitylene]_tot >> [I₂], we can say that [mesitylene] [mesitylene]tot. Prepare a graph of A/([mesitylene][I₂]_tot) versus A/[I₂]_tot and find the equilibrium constant and molar absorptivity of the complex.

[complex] I, + K = [I][mesitylene] Iodine Mesitylene Complex Amax = 332 nm E332 E332 0