sing the following table of prominent ab wavelengths and molar absorptivities f
ID: 697897 • Letter: S
Question
sing the following table of prominent ab wavelengths and molar absorptivities f 41. U for napthalene and Molar Absorption und wavelength nmabsorptivity Compound napthalene 221 286 312 375 130000 9300 290 221 312 375 15000 1200 9000 anthracene anthracene, suggest a wavelength at which anthracene can be determined in the presence of an unknown amount of napthalene and calculate the minimum detectable concentration (MDC) of anthracene (in M) at its optimum wavelength. Assume a cell path length of 1.00 cm and the MDC yields a %T of 97.0. Molar Mass anthracene 166 g.mol- (A) 221 nm, 6.7 × 10-6 (C) 375 nm, 1.8 × 10-4 (B) 286 nm, 1.4 x 10-6 (D) 375 nm, 1.5 x 10-6Explanation / Answer
Anthracene can be conveniently detected in the presence of naphthalene at 375 nm. At 375 nm, anthracene has an appreciable molar absorptivity of 9000 M-1cm-1 (unit of molar absorptivity) whereas naphthalene has a molar absorptivity of 0. This simply means that naphthalene doesn’t absorb any light at 375 nm and the absorbance of a solution containing both anthracene and naphthalene is due to anthracene only.
The solution has a transmittance, %T of 97.0. The absorbance of the solution is given as A = 2 – log (%T) = 0.013. Use Beer’s law to find out the MDC of the solution.
A = *(MDC)*l where = molar absorptivity at 375 nm and l = path length of the solution. Plug in values and obtain
0.013 = (9000 M-1cm-1)*(MDC)*(1.00 cm)
====> MDC = (0.013)/(9000 M-1) = 1.44*10-6 M 1.5*10-6 M.
(D) gives the correct answer.
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