Example (p. 377) – Photon Energies By how many joules is the energy of a molecule increased when it absorbs (a) visible light with a wavelength of 500 nm or (b) infrared radiation with a wavenumber of 1,251 cm -1 ?
Example (p. 379) – Absorbance & Transmittance What absorbance corresponds to 99% transmittance? To 0.10% transmittance?
Beer’s Law: absorbance is proportional to the concentration of light-absorbing molecules in the sample A = bc Beer’s Law for a mixture -
Example (p. 381) – Using Beer’s Law The peak absorbance of 3.16 x M KMNO 4 at 555 nm in a cm pathlength cell in Fig 18-5 is (a) Find the molar absorptivity and percent transmittance of this solution. (b) What would the absorbance be if the pathlength was cm? (c) What would the absorbance be in a cm cell if the concentration was decreased by a factor of 4?
Absorption Spectrum – CoCl 2 R O Y G B V
Example (p. 382) – Finding Concentration from the Absorbance Gaseous ozone has a molar absorptivity of 2700 M -1 cm -1 at the absorption peak near 260 nm in the spectrum below. Find the concentration of ozone (mol/L) in air if a sample has an absorbance of 0.23 in a 10.0 cm cell. Air has negligible absorbance at 260 nm.
Example (p. 383) – How Effective is Sunscreen? What fraction of ultraviolet radiation is transmitted through the sunscreen in the spectrum below at 300 nm?
Using Beer’s Law – Sec 18-4 e.g. measuring NO 2 ¯ in aquarium water
Analysis based on the absorbance of the colored product of this reaction-
Absorbance Spectrum of the Colored Product Absorbance max = __________ nm
Construction of a Calibration Curve (Standard Curve) Table 18-2 SampleAbsorbance at 543 nm in a 1.0 cm cuvette Corrected Absorbance (blank subtracted) Blank0.003 Standards ppm ppm ppm Unknown0.281 Unknown0.277
Calibration Curve for Nitrite Analysis (blank subtracted)
Example (p. 389) – Using the Standard Curve From the data from Table 18-2, find the molarity of nitrite in the aquarium.
The Spectrophotometer – Sec 19-1
Hitachi UV-Vis – U2000
A = total absorbance at wavelength 1 A = total absorbance at wavelength 2 X = molar absorptivity of molecule X at wavelength 1 X = molar absorptivity of molecule X at wavelength 2 Y = molar absorptivity of molecule Y at wavelength 1 Y = molar absorptivity of molecule Y at wavelength 2 Spectrophotometric Analysis of a Mixture – Sec 19-2
A = A X + A Y because Beer’s Law is additive = X b [X] + Y b [Y] A″ = A″ X + A″ Y = ″ X b [X] + ″ Y b [Y]